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White LED Current Regulator with 1x/1.5x
High-Efficiency Charge Pump
[ ( ( 1 . 5 × V ) ) ]
Input Ripple
For LED drivers, input ripple is more important than out-
put ripple. Input ripple depends on the source supply’s
impedance. Adding a lowpass filter to the input further
reduces input ripple. Figure 3 shows a C-R-C filter used
to reduce input ripple to less than 2mV P-P when driving
a 75mA load. Alternately, increasing C IN to 2.2μF or
4.7μF yields input ripple of 17mV P-P or 9mV P-P , respec-
tively, with only a small increase in footprint. The 1x
mode has very low input ripple.
PC Board Layout and Routing
The MAX1570 is a high-frequency switched-capacitor
Component Selection
Typical external component values are shown in Table
2 and Table 3.
Maximum power dissipation occurs around 4.2V on IN
and 4.0V at OUT with maximum load current.
P D = IN ? V OUT + 0 . 4 V × 120 mA = 324 mW
At higher IN voltages this device switches to 1x mode
and power dissipation is lowered.
voltage regulator. For best circuit performance, use a
1 μ F
1 μ F
solid ground plane and place C IN and C OUT as close
to the MAX1570 as possible. Also, place their ground
C1P
C1N
C2P
C2N
tails close together. Connect PGND and GND directly
under the IC to the exposed paddle, and connect C IN
and C OUT as close as possible to PGND. The input
V IN
1 μ F
IN
OUT
4.7 μ F
0 TO 20mA/LED
supply connections should be short; if this is not possi-
ble, an additional input supply filter capacitor (tantalum
or electrolytic) may be required. Refer to the MAX1570
EV kit for a good layout example.
PWM INPUT
R1
14.3k ?
R2
14.3k ?
EN1
EN2
SET
MAX1570
LED1
LED2
LED3
LED4
0 TO 25V
LED5
1 μ F
C1P
C1N
1 μ F
C2P
C2N
C3
1 μ F
R SET
9.09k ?
GND
PGND
V IN
IN
OUT
Figure 2. Dimming Using Filtered-PWM Signal
1 μ F
4.7 μ F
1 μ F
1 μ F
C1P
C1N
C2P
C2N
PWM INPUT
RSET
EN2
EN1
SET
MAX1570
LED1
LED2
LED3
LED4
LED5
V IN
1 ?
2.2 μ F
IN
2.2 μ F
EN1
MAX1570
OUT
LED1
LED2
4.7 μ F
GND
PGND
EN2
LED3
SET
LED4
Figure 1. Dimming Using PWM Signal into EN1
RSET
GND
PGND
LED5
Figure 3. C-R-C Filter Reduces Input Ripple
Chip Information
TRANSISTOR COUNT: 3187
_______________________________________________________________________________________
9
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