MAX770C View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX770C

5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers

Maxim Integrated 

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

Low IQ, Step-Up DC-DC Controllers

drive outputs (EXTH and EXTL) that operate 180° out of

phase (Figures 3a and 3b). In Figure 3b, the resistor in

series with EXTH limits the base current, and EXTL (which

is connected directly to the base) turns the transistor off.

Shutdown Mode

When SHDN is high, the MAX770–MAX773 enter shut-

down mode. In this mode, the internal biasing circuit-

ry is turned off (including the reference) and VOUT

falls to a diode drop below VIN (due to the DC path

from the input to the output). In shutdown mode, the

supply current drops to less than 5µA. SHDN is a

TTL/CMOS logic-level input. Connect SHDN to GND for

normal operation.

The MAX773’s shunt regulator is not disabled in shut-

down mode.

Low-Battery Detector

The MAX773 provides a low-battery comparator that

compares the voltage on LBI to the reference voltage.

When the LBI voltage is below VREF, LBO (an open-

drain output) goes low. The low-battery comparator’s

20mV of hysteresis adds noise immunity, preventing

repeated triggering of LBO. Use a resistor-divider network

between V+, LBI, and GND to set the desired trip voltage

VTRIP. LBO is high impedance in shutdown mode.

__________________Design Procedure

Setting the Output Voltage

To set the output voltage, first determine the mode of

operation, either bootstrapped or non-bootstrapped.

Bootstrapped mode provides more output current

capability, while non-bootstrapped mode reduces the

supply current (see Typical Operating Characteristics).

If a decaying voltage source (such as a battery) is

used, see the additional notes in the Low Input Voltage

Operation section.

Use the MAX770/MAX771/MAX772 unless one or more

of the following conditions applies. If one or more of the

following is true, use the MAX773:

1) An NPN power transistor will be used as the power

switch

2) The LBI/LBO function is required

3) The shunt regulator must accommodate a high

input voltage

4) Preset-output non-bootstrapped operation is

desired—for example, to reduce the no-load

supply current in a 5V to 12V application.

FB

MAX770

MAX771

MAX772

MAX773

GND

Figure 5. Adjustable Output Circuit

R2

VOUT

R1

R1 = 10k TO 500k

( ) R2 = R1 VOUT -1

VREF

VREF = 1.5V

See Table 1 for a summary of operating characteristics

and requirements for the ICs in bootstrapped and non-

bootstrapped modes.

The MAX770–MAX773’s output voltage can be adjust-

ed from very high voltages down to 3V, using external

resistors R1 and R2 configured as shown in Figure 5.

For adjustable-output operation, select feedback resis-

tor R1 in the range of 10kΩ to 500kΩ. R2 is given by:

( ) R2 = (R1) –V–O––U–T-1

VREF

where VREF equals 1.5V.

For preset-output operation, tie FB to GND (this

forces bootstrapped-mode operation for the

MAX770/MAX771/MAX772).

Configure the MAX773 for a preset voltage of 5V, 12V, or

15V by connecting the output to the corresponding

sense input pin (i.e., V5, V12, or V15). FB must be tied to

ground for preset-output operation. Leave all unused

sense input pins unconnected. Failure to do so will cause

an incorrect output voltage. The MAX773 can provide

a preset output voltage in both bootstrapped and non-

bootstrapped modes.

Figures 2 and 3 show various circuit configurations for

bootstrapped/non-bootstrapped, preset/adjustable

operation.

Shunt-Regulator Operation

When using the shunt regulator, connect SGND to ground

and place a 0.1µF capacitor between V+ and SGND, as

close to the IC as possible. Increase C2 to 1.0µF to

improve shunt regulators performance with heavy loads.

Select RSHUNT such that 1mA ≤ ISHUNT ≤ 20mA.

______________________________________________________________________________________ 13

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