LTC3205_15 Datasheet, PDF (12/16 Pages) Linear Technology – Multidisplay LED Controller

LTC3205
APPLICATIO S I FOR ATIO
The efficiency of the LTC3205 depends upon the mode in
which it is operating. Recall that the LTC3205 operates as
a pass switch, connecting V_IN to CPO until one of the LEDs
on the main or sub displays drops out. This feature
provides the optimum efficiency available for a given input
voltage and LED forward voltage. When it is operating as
a switch, the efficiency is approximated by:
η ≡ ^PLED = ^{VLED • ILED} ≅ ^VLED
P_IN V_IN • I_IN V_IN
since the input current will be very close to the LED
current.
At moderate to high output power, the quiescent current
of the LTC3205 is negligible and the expression above is
valid. For example, with V_IN = 3.9V, I_OUT = 20mA • 6 LEDs
and V_LED equal to 3.6V, the measured efficiency is 92.2%,
which is very close to the theoretical 92.3% calculation.
Once an LED drops out, the LTC3205 switches into step-
up mode. Employing the fractional ratio 2:3 charge pump,
the LTC3205 provides more efficiency than would be
achieved with a voltage doubling charge pump.
In 2:3 boost mode, the efficiency is similar to that of a
linear regulator with an effective input voltage of 1.5 times
the actual input voltage. This is because the input current
for a 2:3 fractional charge pump is approximately 1.5
times the load current. In an ideal 2:3 charge pump, the
power efficiency would be given by:
η_IDEAL
≡
P_LED
P_IN
=
V_LED • I_LED
V_IN
•
3
2
I_LED
≅
V_LED
1.5V_IN
Using the RED, GREEN and BLUE Pins with Higher
Currents
The RED, GREEN and BLUE current source pins can be
used at higher current levels to provide features such as a
flash or camera light. Given that the output impedance of
the currrent source is approximately 3.3Ω when in satu-
ration, more compliance voltage will be necessary to
operate the device at higher LED currents. Figure 8 shows
the current source accuracy of the RED, GREEN and BLUE
pins as a function of the pin voltage for various high
current settings.
Programming the I_MS or I_RGB pins for more than 75µA
requires a higher supply voltage to support the extra
current. Figure 9 shows the minimum input supply voltage
required to support various levels of current on the I_MS and
I_RGB pins.
120
V_IN = 3.6V
T_A = 25°C
100
80
60
40
20
I_RGB = 250µA
I_RGB = 200µA
I_RGB = 150µA
I_RGB = 100µA
I_RGB = 50µA
0
0
0.2
0.4
0.6
0.8
1.0
LED PIN VOLTAGE (V)
3205 F08
Figure 8. Compliance Voltage Required to get Higher
LED Currents
3.9
T_A = 25°C
3.7
3.5
I_MS
3.3
3.1
I_RGB
2.9
2.7
2.5
25
50 75 100 125 175 200 225 250
I_MS OR I_RGB CURRENT (µA)
3205 F09
Figure 9. Input Supply Voltage Required to Support
Higher Currents
3205f
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