Datasheet

ADM8843
Rev. C | Page 15 of 16
POWER EFFICIENCY
The ADM8843 power efficiency (η) equations are as follows:
η =
P
OUT
/P
IN
P
IN
= ((V
CC
× I
LOAD
× Gain) + (I
Q
× V
CC
))
P
OUT
= 4 × (V
F
× I
LED
)
where:
I
Q
is the quiescent current of the ADM8843, 2.6 mA.
V
F
is the LED forward voltage.
Gain is the charge pump mode (1×, 1.5×, 2×).
Example 1
The ADM8843 driving four white LED with 20 mA/LED at
V
CC
= 3.4 V (1.5× mode), LED V
F
= 4.5 V.
P
IN
= ((V
CC
× I
LOAD
× Gain) + (V
CC
× I
Q
))
P
IN
= ((3.4 × 80 mA × 1.5) + (3.4 × 2.6 mA))
P
IN
= ((0.408) + (0.00884))
P
IN
= 0.41684
P
OUT
= 4(V
F
× I
LED
)
P
OUT
= 4(4.5 V × 20 mA)
P
OUT
= 0.36
η =
P
OUT
/P
IN
η = 0.36/0.41684
η = 87 %
V
OUT
FB1
FB2
FB3
FB4
V
CC
= 3.4V
CTRL1
CTRL2
GND
ADM8843
R
SET
7.32kΩ
I
SET
05050-019
V
CC
I
IN
I
LOAD
Figure 30. Charge Pump Power Efficiency Diagram, Example 1
Example 2
The ADM8843 driving four white LED with 20 mA/LED at
V
CC
= 3.4 (1.5× mode), LED V
F
= 3.6 V.
P
IN
= ((V
CC
× I
LOAD
× Gain) + (V
CC
× I
Q
))
P
IN
= ((3.4 × 80 mA × 1.5) + (3.4 × 2.6 mA))
P
IN
= ((0.408) + (0.00884))
P
IN
= 0.41684
P
OUT
= 4(V
F
× I
LED
)
P
OUT
= 4(3.6 V × 20 mA)
P
OUT
= 0.288
η =
P
OUT
/P
IN
η = 0.288/0.41684
η = 70 %