Datasheet

ADM8845
Rev. C | Page 17 of 20
POWER EFFICIENCY
The ADM8845 power efficiency (η) equations are
η = P
OUT
/P
IN
P
IN
= ((V
CC
× I
LOAD
× Gain) + (I
Q
× V
CC
))
P
OUT
= 6 × (V
F
× I
LED
)
where:
I
Q
is the quiescent current of the ADM8845, 2.6 mA.
V
F
is the LED forward voltage.
Gain is equal to the charge pump mode (1
×, 1.5×, 2×).
FB1
FB2
FB3
FB4
FB5
FB6
V
OUT
CTRL1
CTRL2
R
SET
I
SET
ADM8845
04867-0-033
V
CC
3.4V
V
CC
GND
I
LOAD
I
IN
Figure 34. Charge Pump Power Efficiency Diagram
Examples 1 and 2 show calculations of the ADM8845 power
efficiency. See Figure 34 as well.
Example 1
The ADM8845 driving six 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 × 120 mA × 1.5) + (3.4 × 2.6 mA))
P
IN
= ((0.612) + (0.00884))
P
IN
= 0.62084
P
OUT
= 6 × (V
F
× I
LED
)
P
OUT
= 6 × (4.5V × 20 mA)
P
OUT
= 0.54
η = P
OUT
/P
IN
η = 0.54/0.62084
η = 87%
Example 2
The ADM8845 driving six 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 × 120 mA × 1.5) + (3.4 × 2.6 mA))
P
IN
= ((0.612) + (0.00884))
P
IN
= 0.62084
P
OUT
= 6 × (V
F
× I
LED
)
P
OUT
= 6 × (3.6 V × 20 mA)
P
OUT
= 0.432
η = P
OUT
/P
IN
η = 0.432/0.62084
η = 70%