MAX747
High-Efficiency PWM, Step-Down
P-Channel DC-DC Controller
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PIN NAME FUNCTION
1 LBI Input to the internal low-battery comparator. Tie to V+ or GND if not used.
2 SS
Soft-start limits start-up surge currents. On power-up, it charges the soft-start capacitor, slowly raising the
peak current limit to the level set by the sense resistor.
3 REF
2V reference output that can source 100µA for external loads. Bypass with 0.22µF. The reference is
disabled in shutdown mode.
4 SHDN
Active-high TTL/CMOS logic-level input. In shutdown mode, V
OUT
= 0V and the supply current is reduced
to 20µA.
5 FB
Feedback input for adjustable-output operation. Connect to GND for fixed +5V output. Use a resistor
divider network to adjust the output voltage. See the section
Setting the Output Voltage
.
6 CC
Compensation capacitor. AC compensation input for the error amplifier. Connect a capacitor between CC
and GND for fixed +5V output operation. See
Compensation Capacitor
section.
7 AV+ Quiet supply voltage for sensitive analog circuitry. A bypass capacitor is not required for AV+.
8 OUT
Output voltage sense input. Connects to internal resistor divider. Leave unconnected for adjustable output.
Bypass to AGND with a 0.1µF capacitor close to the IC.
9 CS Negative input to the current-sense amplifier. Connect the current-sense resistor (R
SENSE
) from V+ to CS.
10 AGND Quiet analog ground
11 EXT
Power MOSFET gate drive output that swings between V+ and GND. EXT is not protected against short
circuits to V+ or AGND.
12 V+ High-current supply voltage for the output driver
13 GND High-current ground return for the output driver
14 LBO
Low-battery output is an open-drain output that goes low when LBI is less than 2V. Connect to V+ through
a pull-up resistor. Leave floating if not used. LBO is disabled in shutdown mode.
______________________________________________________________Pin Description
____________________Getting Starting
Figure 1a shows the 5V output 11.4W standard
application circuit and Figure 1b shows the 3.3V output
7.5W standard application circuit. Most applications
will be served by these circuits. To learn more about
component selection for particular applications, refer to
the
Design Procedure
section. To learn more about the
operation of the MAX747, refer to the
Detailed
Description.
_______________Detailed Description
The MAX747 monolithic, CMOS, step-down switch-
mode power-supply controller drives external
P-channel FETs. It uses a unique current-mode pulse-
width-modulating (PWM) control scheme that results in
high efficiency over a wide range of load currents, tight
output voltage regulation, excellent load- and line-
transient response, and low noise. Efficiency at light
loads is further enhanced by a proprietary Idle-Mode
switching control scheme that skips oscillator cycles in
order to reduce switching losses.
