Table 2. p1 and p2, ldo output voltage selection – Rainbow Electronics MAX8645Y User Manual
Page 12
MAX8645X/MAX8645Y
1x/1.5x/2x White LED Charge Pumps with Two
LDOs in 4mm x 4mm TQFN
12
______________________________________________________________________________________
Input Ripple
For LED drivers, input ripple is more important than out-
put ripple. Input ripple is highly dependent on the
source supply’s impedance. Adding a lowpass filter to
the input further reduces input ripple. Alternately,
increasing C
IN
to 22µF cuts input ripple in half with only
a small increase in footprint. The 1x mode always has
very low input ripple.
Typical operating waveforms shown in the Typical
Operating Characteristics show input ripple current in
1x, 1.5x, and 2x modes.
LDO Output Voltage Selection (P1 and P2)
As shown in Table 2, the LDO output voltages, LDO1
and LDO2 are pin programmable by the logic states of
P1 and P2. P1 and P2 are tri-level inputs: IN, open, and
GND. The input voltage, V
IN
, must be greater than the
selected LDO1 and LDO2 voltages. The logic states of
P1 and P2 can be programmed only during ENLDO
low. Once the LDO_ voltages are programmed, their
values do not change by changing P1 or P2 during
ENLDO high.
Component Selection
Use only ceramic capacitors with an X5R, X7R, or better
dielectric. See Table 3 for a list of recommended parts.
Connect a 1µF ceramic capacitor between LDO1 and
GND, and a second 1µF ceramic capacitor between
LDO2 and GND for 200mA applications. The LDO out-
put capacitor’s (C
LDO
) equivalent series resistance
(ESR) affects stability and output noise. Use output
capacitors with an ESR of 0.1
Ω or less to ensure stability
and optimum transient response. Connect C
LDO_
as
close as possible to the MAX8645X/MAX8645Y to mini-
mize the impact of PCB trace inductance.
MAX8645X
MAX8645Y
1
μF
10
μF
0.01
μF
10
μF
PIN
IN
GND
P1
P2
OUT
MAIN
FLASH
OUTPUT
UP TO 240mA
1
μF
1
μF
LDO1
LDO2
C2P
C2N
1
μF
C1P
C1N
REFBP
SETM
SETF
PGND
ON/OFF AND
VOLTAGE
SELECTION
INPUT
2.7V TO 5.5V
ENM2
ENM1
ENF
ENLDO
M1
M2
M3
M4
M5
M6
F1
F2
4.12k
Ω
6.81k
Ω
Figure 4. Schematic for When Fewer than 8 LEDs Are Acceptable
MAX8645X
MAX8645Y
P1
P2
LDO1 (V) LDO2 (V)
LDO1 (V) LDO2 (V)
IN
IN
3.3
1.8
2.8
2.6
IN
OPEN
3.0
1.5
2.8
2.8
IN
GND
2.8
1.5
2.9
1.5
OPEN
IN
3.3
1.5
2.6
1.9
OPEN
OPEN
2.6
1.8
2.6
2.6
OPEN
GND
2.6
1.5
2.8
1.9
GND
IN
3.0
1.8
2.9
1.8
GND
OPEN
2.8
1.8
2.9
1.9
GND
GND
2.5
1.8
2.9
2.9
Table 2. P1 and P2, LDO Output Voltage
Selection