connected to the output voltage (-5V, -12V, -15V). In

nonbootstrapped operation, OUT is connected to

ground, and EXT now swings from V+ to ground.

At high input-to-output differentials, it may be neces-

sary to use nonbootstrapped mode to avoid the 21V V+

mum rating of the external transistor. At intermediate

voltages and currents, the advantages of bootstrapped

vs. nonbootstrapped operation are slight. When input

voltages are less than about 4V, always use the boot-

strapped circuit.

Shutdown and Quiescent Current

The MAX774/MAX775/MAX776 are designed to save

power in battery-powered applications. A TTL/CMOS

logic-level shutdown input (SHDN) has been provided

for the lowest-power applications. When shut down

(SHDN = V+), most internal bias current sources and

the reference are turned off so that less than 5µA of

current is drawn.

In normal operation, the quiescent current will be less

than 100µA. However, this current is measured by forc-

ing the external switch transistor off. Even with no load,

in an actual application, additional current will be

drawn to supply the feedback resistors’ and the diode’s

and capacitor’s leakage current. Under no-load condi-

tions, you should see a short current pulse at half the

peak current approximately every 100ms (the exact

period depends on actual circuit leakages).

EXT Drive Voltages

EXT swings from OUT to V+ and provides the drive out-

put for an external power MOSFET. When using the on-

chip feedback resistors for the preset output voltages,

the voltage at OUT equals the output voltage. When

using external feedback resistors, OUT may be tied to

Always observe the V+ to OUT absolute maximum rat-

ing of 21V. For V+ to output differentials greater than

21V, OUT must be tied to a potential more positive than

the output and, therefore, the output voltage must be

set with an external resistor divider.

In nonbootstrapped operation with low input voltages

(<4V), tie OUT to a negative voltage to fully enhance the

external MOSFET. Accomplish this by creating an inter-

__________________Design Procedure

Setting the Output Voltage

The MAX774/MAX775/MAX776 are preset for -5V, -12V,

and -15V output voltages, respectively; however, they

may also be adjusted to other values with an external

voltage divider. For the preset output voltage, connect

FB to REF and connect OUT to the output (Figure 3). In

this case, the output voltage is sensed by OUT.

For an adjustable output (Figures 3 and 4), connect an

external resistor divider from the output voltage to FB,

and from FB to REF. In this case, the divided-down out-

put voltage is sensed via the FB pin.

There are three reasons to use the external resistor divider:

1) An output voltage other than a preset value is

desired.

2) The input-to-output differential exceeds 21V.

See Figures 3 and 4 for adjustable operation. The

impedance of the feedback network should be low

enough that the input bias current of FB is not a factor.

For best efficiency and precision, allow 10µA to flow

150k

Ω. Then calculate R2 as follows:

R2

___ = _______

R1

µA)

______

R2

Choosing an Inductor

Practical inductor values range from 10µH to 50µH.

The maximum inductor value is not particularly critical.

For highest current at high

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

______________________________________________________________________________________

11

Figure 5. Connection Using Zener Diode to Boost Base Drive

R1

MAX774

MAX775

MAX776

OUT

GND

FB

R2

8

2

1

REF

4

0.1

µF

0.1

µF

6V