5V/12V/15V or Adjustable, High-Efficiency,

8

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_______________Detailed Description

The MAX770–MAX773 are BiCMOS, step-up, switch-

mode power-supply controllers that provide preset 5V,

12V, and 15V output voltages, in addition to adjustable-

output operation. Their unique control scheme com-

bines the advantages of pulse-frequency modulation

(low supply current) and pulse-width modulation (high

efficiency with heavy loads), providing high efficiency

over a wide output current range, as well as increased

output current capability over previous PFM devices.

In addition, the external sense resistor and power

transistor allow the user to tailor the output current

capability for each application. Figure 1 shows the

MAX770–MAX773 block diagram.

The MAX770–MAX773 offer three main improvements

over prior pulse-skipping control solutions: 1) the con-

verters operate with tiny (5mm height and less than

9mm diameter) surface-mount inductors due to their

300kHz switching frequency; 2) the current-limited PFM

control scheme allows 87% efficiencies over a wide

range of load currents; and 3) the maximum supply

current is only 110µA.

The MAX773 can be configured to operate from an

internal 6V shunt regulator, allowing very high input/out-

put voltages. Its output can be configured for an

adjustable voltage or for one of three fixed voltages

(5V, 12V, or 15V), and it has a power-fail comparator for

low-battery detection.

All devices have shutdown capability, reducing the

supply current to 5µA max.

Bootstrapped/Non-Bootstrapped Modes

Figures 2 and 3 show standard application circuits for

bootstrapped and non-bootstrapped modes. In boot-

strapped mode, the IC is powered from the output

ground, providing more switch gate drive and thus

reducing the transistor’s on resistance.

In non-bootstrapped mode, the IC is powered from the

input voltage (V+) and operates with minimum supply

current. In this mode, FB is the output voltage sense

point. Since the voltage swing applied to the gate of the

external power transistor is reduced (the gate swings

from V+ to ground), the power transistor’s on resistance

increases at low input voltages. However, the supply

current is also reduced because V+ is at a lower volt-

age, and because less energy is consumed while

charging and discharging the external MOSFET’s gate

capacitance. The minimum input voltage for the

MAX770–MAX773 is 3V when using external feedback

resistors. With supply voltages below 5V, bootstrapped

mode is recommended.

Note: When using the MAX770/MAX771/MAX772 in

non-bootstrapped mode, there is no preset output

operation because V+ is also the output voltage

sense point for fixed-output operation. External

resistors must be used to set the output voltage.

Use 1% external feedback resistors when operating

in adjustable-output mode (Figures 2c, 2d, 3b, 3d, 3e)

to achieve an overall output voltage accuracy of ±5%.

The MAX773 can be operated in non-bootstrapped

mode without using external feedback resistors

because V+ does not act as the output voltage sense

point with preset-output operation. To achieve high-

est efficiency, operate in bootstrapped mode when-

ever possible.

MAX773 Shunt-Regulator Operation

The MAX773 has an internal 6V shunt regulator that

allows the device to step up from very high input

voltages (Figure 4).

PIN

NAME

FUNCTION

MAX770

MAX771

MAX772

MAX773

—

13

EXTH

—

12

EXTL

Low-level gate/base drive for external power transistor. Connect to the gate of an external

N-channel MOSFET or to the base of an external NPN transistor.

—

14

V15

Leave unconnected for adjustable-output operation

_________________________________________________Pin Description (continued)

High-level gate/base drive for external power transistor. Connect to EXTL when using an external

EXTH to the base of the NPN to set the maximum base-drive current.