Rainbow Electronics MAX1709 - Manual (Page 7)

MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
_______________________________________________________________________________________ 7
SHUTDOWN WITH SOFT-START
(C SS = 0.01 µ F)
MAX1709-17
5V
0
1ms/div
V OUT
2V/div
V ONA
5V/div
4A
0
4V
I IN
2A/div
6A
2V
C SS = 0.01 µ F
R OUT = 5 Ω
V ONB = V OUT
2A
SHUTDOWN WITHOUT SOFT-START
MAX1709-18
5V
0
1ms/div
V OUT
2V/div
V ONA
5V/div
4A
0
4V
I IN
2A/div
6A
2V
C SS = 0
R OUT = 5 Ω
V ONB = V OUT
2A
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
Pin Description
PIN NAME FUNCTION
1 ONA On-Control Input. When ONA = high or ONB = low, the device turns on (Table 1).
2, 3, 4 LX
Drain of N-Channel Power Switch. Connect pins 2, 3, and 4 together. Connect external
Schottky diode from LX to OUT.
5, 8 GND Ground
6 SS/LIM
Soft-Start and/or Current-Limit Input. Connect a capacitor from SS/LIM to GND to control the
rate at which the device reaches current limit (soft-start). To reduce the current limit from the
preset values, connect a resistor from SS/LIM to GND (see Design Procedure). During
shutdown, this pin is internally pulled to GND to discharge the soft-start capacitor.
7 REF
1.26V Voltage Reference Output. Bypass with a 0.22
µ
F capacitor to GND. Maximum REF load
is 50
µ
A.
9 OUT
Output Voltage Sense Input. The device is powered from OUT. Bypass with a 0.1
µ
F to PGND
with less than 5mm trace length. Connect a 2
Ω
series resistor from the output filter capacitor to
OUT (Figure 1).
10 FB
DC-DC Converter Feedback Input. Connect FB to GND for internally set output voltage (see
3.3
/5 pin description). Connect a resistor-divider from the output to set the output voltage in
the +2.5V to +5.5V range. FB regulates to +1.25V (Figure 4).
11, 12, 13 PGND Power Ground, Source of N-Channel Power MOSFET Switch
14 3.3/5
Output Voltage Selection Pin. When FB is connected to GND, the regulator uses internal
feedback to set the output voltage. 3.3
/5 = low sets output to 3.3V; 3.3/5 = high sets output to
5V. If an external divider is used at FB, connect 3.3 /5 to ground.
15 CLK
Clock Input for the DC-DC Converter. Connect to OUT for internal oscillator. Optionally, drive
with an external clock for external synchronization.
16 ONB Shutdown Input. When ONB = high and ONA = low, the device turns off (Table 1).