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doc/LCD-backlight-driver article written
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 10 May 2020 22:48:39 +0000
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70
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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1 I, Mother Mychaela, have a deep desire to build my own GSM cellphone handset
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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2 that would serve as a published-source replacement for my current Pirelli
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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3 DP-L10, which is laden with unwanted and undocumented extra non-GSM components
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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4 and for which there are no schematics. I already know what kind of display I
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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5 wish to use in my dream FreeCalypso Libre Dumbphone: a 2.0" 176x220 pixel TFT
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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6 color LCD, strictly transmissive, requiring a backlight - same principal class
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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7 of LCD as in the Pirelli DP-L10, but stepping up in size from Pirelli's 128x128
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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8 to 176x220 pixels. There are many vendors who make suitable LCD modules, and
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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9 there are two specific candidate modules already in use at FreeCalypso HQ as
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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10 part of various prototype rigs.
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11
b1b027efce8e doc/LCD-backlight-driver article written
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12 The backlight is implemented in exactly the same way on all candidate 2.0"
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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13 176x220 pixel TFT LCD modules I have looked at: it consists of 3 white LEDs,
b1b027efce8e doc/LCD-backlight-driver article written
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14 joined together either at the anode or at the cathode, with the opposite
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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15 terminal brought out separately for each of the 3 LEDs, supporting an
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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16 arrangement where the 3 LEDs are driven in parallel rather than in series.
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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17 Each of the 3 LEDs needs to have about 15 mA flowing through it for maximum
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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18 display brightness; lower LED currents will produce lower display brightness,
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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19 but going significantly above 15 mA would be bad - too much current would burn
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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20 out the LEDs.
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21
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Mychaela Falconia <falcon@freecalypso.org>
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22 Exactly how should this backlight be driven in our FreeCalypso Libre Dumbphone
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Mychaela Falconia <falcon@freecalypso.org>
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23 design? In this article I am going to look at some obvious and less obvious
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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24 ways to drive backlight LEDs, and then present my own novel way (novel in that
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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25 I haven't seen it used in any existing design or seen it recommended anywhere)
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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26 which has already been implemented on our current Luna development platform.
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27
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28 The trivial way: VBAT and series resistors
b1b027efce8e doc/LCD-backlight-driver article written
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29 ==========================================
b1b027efce8e doc/LCD-backlight-driver article written
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30
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Mychaela Falconia <falcon@freecalypso.org>
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31 The most trivial way to drive a backlight LED or a parallel group of such LEDs
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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32 in a mobile phone whose ultimate power source is a single-cell Li-ion battery
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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33 would be to put a current limiting resistor in series with each LED, and then
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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34 connect each LED+resistor set between VBAT and GND, i.e., across battery
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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35 terminals. (Of course a transistor would also need to be inserted somewhere to
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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36 act as on/off switch, turning the backlight on only when it is needed.)
b1b027efce8e doc/LCD-backlight-driver article written
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37
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Mychaela Falconia <falcon@freecalypso.org>
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38 With this trivial arrangement the value of the series resistors (one in series
b1b027efce8e doc/LCD-backlight-driver article written
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39 with each LED) would need to be calculated as follows:
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40
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41 R = (VBAT_max - Vled) / Iled_max
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42
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43 where VBAT_max is the maximum allowed battery voltage (4.2 V for typical Li-ion
b1b027efce8e doc/LCD-backlight-driver article written
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44 batteries), Vled is the voltage drop across a backlight LED, and Iled_max is the
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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45 maximum current that should ever flow through each individual LED.
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46
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47 The big problem with this trivial LED driver approach is that the display
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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48 backlight will glow at its maximum brightness only when the battery is at its
b1b027efce8e doc/LCD-backlight-driver article written
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49 peak charge, and will dim as the battery discharges. Why so? The series
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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50 resistor value would need to be set per the equation above in order to avoid
b1b027efce8e doc/LCD-backlight-driver article written
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51 damage to the backlight LEDs (the current through each LED must not exceed
b1b027efce8e doc/LCD-backlight-driver article written
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52 Iled_max at the highest battery voltage), but then as the battery discharges,
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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53 the voltage across each LED series resistor will decline (VBAT - Vled, with
b1b027efce8e doc/LCD-backlight-driver article written
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54 Vled assumed to be constant), and the current through the resistor (and thus
b1b027efce8e doc/LCD-backlight-driver article written
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55 through the LED as well) will decline proportionally.
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56
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57 How do LCD backlights in mainstream commercial phones behave in this regard?
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Mychaela Falconia <falcon@freecalypso.org>
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58 I have a disassembled Pirelli DP-L10 phone (bare motherboard with the LCD and
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59 the keypad still attached) which I have hacked up to be powered by a lab bench
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60 power supply instead of the usual battery, and I did an experiment with it: I
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61 powered up this Pirelli motherboard with my bench supply, running Pirelli's
b1b027efce8e doc/LCD-backlight-driver article written
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62 original firmware, I got it into a state where both LCD and keypad backlights
b1b027efce8e doc/LCD-backlight-driver article written
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63 are on (press any keypad button to turn them back on when the fw turns them off
b1b027efce8e doc/LCD-backlight-driver article written
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64 by timeout), I turned the voltage knob on the power supply up and down, and I
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65 observed the brightness of both LCD and keypad backlights.
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66
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67 Observation: Pirelli's keypad backlight does get noticeably brighter or dimmer
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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68 as VBAT goes up and down, indicating that they do use the trivial driver circuit
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69 for this one (from fw perspective, Pirelli's keypad backlight is driven or at
b1b027efce8e doc/LCD-backlight-driver article written
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70 least controlled with Iota LED-B), but the LCD brightness stays exactly the same
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71 as VBAT ranges from the 4.2 V Li-ion maximum to the low-battery emergency
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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72 shut-off voltage (about 2.8 V) at which the Iota VRPC block involuntarily shuts
b1b027efce8e doc/LCD-backlight-driver article written
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73 down the entire Calypso subsystem.
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74
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75 It is not clear exactly how Pirelli's LCD backlight driver circuit is
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Mychaela Falconia <falcon@freecalypso.org>
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76 implemented. There is a component on their motherboard near the LCD connector
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77 marked as A3-90E - it might be the LED driver - and there is another little
b1b027efce8e doc/LCD-backlight-driver article written
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78 component next to it that looks like an inductor, suggesting some kind of boost
b1b027efce8e doc/LCD-backlight-driver article written
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79 converter. There is no documentation for Pirelli's Giantplus GPM526A0 LCD
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Mychaela Falconia <falcon@freecalypso.org>
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80 module, but it seems to have just two wires for the backlight, suggesting that
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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81 the two backlight LEDs (this LCD module has 2 backlight LEDs rather than 3) may
b1b027efce8e doc/LCD-backlight-driver article written
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82 be wired in series (not parallel), in which case a boost converter would be
b1b027efce8e doc/LCD-backlight-driver article written
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83 absolutely required.
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84
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85 Boost to 5V, then fixed series resistors
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86 ========================================
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87
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88 The available schematics for Motorola C139 and C155 phones depict an LCD
b1b027efce8e doc/LCD-backlight-driver article written
Mychaela Falconia <falcon@freecalypso.org>
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89 backlight driver circuit that seemed bizarre to me at first: they take VBAT,
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90 boost it up to constant 5V with a step-up charge pump (RT9361A on C139
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91 schematics, REG710NA-5 on C155 schematics), and feed that 5V to their LCD
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92 module, which presumably expects fixed 5V backlight power and internally
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93 contains a fixed resistor in series with each LED.
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94
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95 This approach certainly accomplishes the goal of constant LCD backlight
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96 brightness irrespective of battery state of charge, but it does so at a huge
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97 cost in terms of efficiency. Both RT9361A and REG710NA-5 are step-up charge
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98 pumps, and they work by doubling the current draw. If we were to use the same
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99 arrangement for our LCD backlight (3 LEDs, each needing 15 mA), then for 45 mA
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100 of current flowing through the LEDs, 90 mA will be drawn from the battery.
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101 These are not "smart" boost converters that draw less input current as their
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102 input voltage goes up (for same I*V power), instead the input current is an
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103 almost constant 2x the output current, thus the overall efficiency gets very
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104 poor at higher battery voltages.
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105
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106 I strongly dislike this approach for its wastefulness, hence I sought another
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107 way.
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108
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109 My novel 3.5V LDO approach
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110 ==========================
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111
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112 Datasheets for the LCD modules I am working with specify the drop voltage across
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113 each of the 3 backlight LEDs as 3.2V. The table of battery voltage thresholds
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114 (mapping VBAT to battery state of charge percentages) inside Pirelli's firmware
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115 (located and extracted via thorough reverse eng) has these mappings at the lower
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116 end:
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117
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118 3719 20
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119 3688 15
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120 3663 10
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121 3539 5
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122 3370 0
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123
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124 These numbers make it clear that a battery voltage around 3.5 to 3.6 V means
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125 that the battery is near empty; combining this "low battery" number with the
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126 datsheet-stated LED drop voltage of 3.2 V gave me this idea: what if we feed
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127 VBAT to a 3.5V LDO regulator and use this LDO output as the backlight power
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128 source, with the LED series resistor values computed for 3.5 V supply? This
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129 approach would produce constant LCD brightness for the wide VBAT range from
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130 just above 3.5 V (the LDO regulator's dropout is very low) to 4.2 V or above,
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131 without doubling the current draw (for 45 mA flowing through the LEDs,
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132 approximately the same 45 mA will be drawn from the battery), with the only
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133 anticipated penalty being a possible sharp drop-off in LCD brightness when the
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134 battery gets critically low.
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135
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136 When I was designing my FC Luna UI development platform (an LCD add-on to the
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137 existing historical third-party Caramel board), I sought to test this idea
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138 empirically. But before actually building this Luna LCD board, I fortunately
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139 had the foresight to measure the actual voltage drop across the backlight LEDs,
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140 rather than blindly rely on the datasheet spec of 3.2 V. Back in 2018 I had
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141 tested my chosen LCD modules in a standalone environment without Calypso: I had
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142 them switched into 8-bit microprocessor bus interface mode (IM0 pin strapping)
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143 and I drove them with an FT2232D adapter using FTDI's MCU host bus emulation
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144 mode. I still have the two hardware setups (LCD modules from two different
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145 vendors) I had put together back then; the backlight power source in these
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146 setups is USB 5V, with 110 or 120 ohm LED series resistors. I took the one
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147 setup on which the point between each LED cathode and the connected series
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148 resistor is easily accessible for probing, and I measured the voltage at that
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149 point, to see how the overall 5V gets split between the drop across the LED and
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150 the drop across the resistor.
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151
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152 The answer was somewhat unexpected: the voltage drop across each LED turned out
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153 to be somewhere around 2.9 V, as opposed to the 3.2 V datasheet number. This
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154 difference in the LED forward drop voltage does highlight one major weakness of
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155 my close-to-Vled LDO approach: by setting the backlight fixed voltage so close
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156 to the expected forward drop voltage of the actual LEDs, I am making my circuit
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157 extremely sensitive to slight variations in that forward drop voltage. If I
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158 had populated LED series resistors on my Luna LCD board based on the 3.2 V
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159 assumption (assuming 300 mV drop across each resistor), then the current flowing
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160 through the LEDs would be double of my design intent (with Vled = 2.9 V, the
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161 voltage drop across each resistor becomes 600 mV), possibly burning out the
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162 LEDs! In contrast, a circuit in which each LED+resistor set is driven with a
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163 much higher voltage (meaning a larger voltage drop across the resistor and a
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164 larger resistor value) is much less sensitive to variations in Vled, producing
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165 much less resulting variation in Iled.
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166
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167 I ended up building my Luna LCD board with my 3.5V LDO backlight LED driver
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168 circuit intact, but I populated 38.3 ohm series resistors instead of my
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169 originally intended 20 ohm value. The resulting circuit works well in practice:
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170 the LDO puts out a very precise 3.5 V for any higher VBAT input, the LCD
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171 backlight is bright and visually pleasing, the measured voltage drop across the
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172 resistors with the backlight on is right about 600 mV, meaning that the 2.9 V
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173 LED forward drop voltage hasn't changed, and the current flowing through each
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174 LED is in the desired 15-16 mA target range. The LDO regulator's enable input
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175 also conveniently serves as the backlight on/off control, driven by Calypso
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176 GPIO 9 in the complete Luna setup. (Calypso MCSI is used only in modem configs,
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177 not in handset configs, thus MCSI pins become GPIOs in the latter, available for
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178 functions like LCD backlight control.)
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179
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180 I then set out to test what happens when the VBAT input to my Luna LCD backlight
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181 driver falls below 3.5 V. At lower voltages the LDO regulator becomes
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182 essentially a pass-through, with the low battery voltage applied almost directly
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183 to each LED+resistor set. The current flowing through the LEDs falls
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184 accordingly, but the question to be answered was what happens to the visual
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185 readability of the LCD. The answer turned out to be very positive: I set my
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186 VBAT-generating lab bench power supply as low as 2.8 V (the emergency shut-off
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187 voltage for Iota VRPC), and while the display naturally gets very dimmed, it is
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188 still readable! This finding tells us that my 3.5V LDO approach does not
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189 present the problem I was afraid of (the display going totally dark in
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190 critically low battery scenarios when the rest of the phone still has some life
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191 left), and the only remaining concern with this approach is the extremely high
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192 sensitivity to variations in LED forward drop voltage.
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193
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194 Where to go from here
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195 =====================
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196
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197 If I ever get as far as actually building my desired FreeCalypso dream phone,
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198 what LCD backlight driver circuit should I use? Should I keep the 3.5V LDO
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199 circuit that appears to work OK in our current Luna setup, or would I be heading
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200 into trouble with LED forward drop voltage variations? I *really* dislike the
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201 wastefulness of the seemingly-mainstream approach (boost converter to a higher
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202 voltage, then series resistors based on that higher voltage), but I don't know
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203 of any better alternative. If someone with better EE knowledge can suggest a
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204 non-wasteful approach that would eliminate or at least reduce Vled sensitivity,
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205 it would be great, otherwise I will have to stick with my current approach and
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206 hope for the best.
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207
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208 I also desire to add PWM control to this LCD backlight, so that the 45 mA
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209 brightness will be the available maximum, rather than required at all times.
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210 The plan I have in mind is to insert a transistor between the cathode joining
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211 point (where either the 3 LED cathodes or the 3 resistors connected to these
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212 cathodes join) and GND, controlled by Calypso PWL output. Unfortunately this
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213 approach would be difficult to prototype in our current Luna environment
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214 because Calypso LT/PWL output is not easily accessible on the Caramel board: it
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215 does come out of the core module, but it goes to an on-board transistor for an
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216 on-board indicator LED, and does not go to any header pins or test points.