used in a bootstrapped circuit (Figure 5), undervoltage

lockout prevents start-up at low input voltages; but

once started, operation can continue down to a lower

voltage that depends on the load.

There is no undervoltage lockout when the internal feed-

back resistors are used (Figure 2), and special circuitry

guarantees start-up at 2.0V. The start-up circuitry fixes

the duty cycle at 50% until V+ is driven to 2.5V, above

which the normal control system takes over.

Shutdown Mode

The MAX761/MAX762 enter shutdown mode when

SHDN is high. In this mode, the internal biasing circuitry

V+ minus a diode drop (due to the DC path from the

input to the output). In shutdown mode, the supply cur-

rent drops to less than 5µA. SHDN is a TTL/CMOS logic

level input. Connect SHDN to GND for normal operation.

LBO is high impedance during shutdown.

Modes of Operation

When delivering high output currents, the MAX761/

MAX762 operate in CCM. In this mode, current always

flows in the inductor, and the control circuit adjusts the

switch’s duty cycle on a cycle-by-cycle basis to maintain

regulation without exceeding the switch-current capabili-

ty. This provides excellent load-transient response and

high efficiency.

In discontinuous-conduction mode (DCM), current

through the inductor starts at zero, rises to a peak value,

then ramps down to zero on each cycle. Although effi-

ciency is still excellent, the switch waveforms contain

ringing (the inductor's self-resonant frequency). This

ringing is normal and poses no operational problems.

Low-Battery Detector

The MAX761/MAX762 provide a low-battery comparator

that compares the voltage on LBI to the 1.5V reference

open-drain output) goes low. The low-battery compara-

tor’s 20mV of hysteresis adds noise immunity, prevent-

ing repeated triggering of LBO. Use a resistor-divider

network between V+, LBI, and GND to set the desired

ignored and LBO is high impedance. The value of

resistor R3 should be no larger than 500k

Ω to ensure

the LBI leakage current does not cause inaccuracies in

__________________Design Procedure

Setting the Output Voltage

The MAX761/MAX762’s output voltage can be adjusted

from 5V to 16.5V using external resistors R1 and R2

configured as shown in Figures 3 and 5. For adjustable-

output operation, select feedback resistor R1 in the

10k

Ω to 250kΩ range. Higher R1 values within this

range give lowest supply current and best light-load

efficiency. R2 is given by:

Note: Tie FB to GND for fixed-output operation

(bootstrapped mode only).

12V/15V or Adjustable, High-Efficiency,

_______________________________________________________________________________________

9

Figure 4a. CCM, Heavy Load Current Waveform (500mA/div)

Figure 4b. Light/Medium Load Current Waveform (500mA/div)

1A

500mA

1A

500mA

0A