LTC3205 LINER [Linear Technology], LTC3205 Datasheet - Page 12

• Manufacturer Part Number: LTC3205
• Description: Multidisplay LED Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3205
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
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:
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
and V
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
12
LED
Figure 8. Compliance Voltage Required to get Higher
P
P
LED
equal to 3.6V, the measured efficiency is 92.2%,
IN
120
100
80
60
40
20
0
V
0
V
T
LED
V
A
IN
= 25 C
IN
= 3.6V
U
0.2
•
•
I
I
IN
LED
LED Currents
LED PIN VOLTAGE (V)
IN
U
0.4
IN
= 3.9V, I
to CPO until one of the LEDs
V
V
LED
IN
0.6
W
I
I
I
I
I
RGB
RGB
RGB
RGB
RGB
OUT
= 250 A
= 200 A
= 150 A
= 100 A
= 50 A
0.8
= 20mA • 6 LEDs
3205 F08
1.0
U
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:
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 3.3 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
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
I
RGB
IDEAL
pins.
Figure 9. Input Supply Voltage Required to Support
3.9
3.7
3.5
3.3
3.1
2.9
2.7
2.5
P
P
LED
IN
25
T
A
= 25 C
50
MS
V
V
I
Higher Currents
IN
MS
LED
75
or I
OR I
•
100
3
2
•
RGB
RGB
I
I
LED
LED
125
CURRENT ( A)
pins for more than 75 A
I
MS
175
1 5
V
.
200
LED
I
RGB
V
IN
225
3205 F09
250
MS
and
3205f