switch is OFF), the range of input and output voltages is

limited to a 21V absolute maximum differential voltage.

When output voltages more negative than -16V are

required, substitute the MAX764/MAX765/MAX766 with

Maxim’s MAX774/MAX775/MAX776 or MAX1774, which

use an external switch.

PFM Control Scheme

The MAX764/MAX765/MAX766 use a proprietary, cur-

rent-limited PFM control scheme that blends the best

features of PFM and PWM devices. It combines the

ultra-low supply currents of traditional pulse-skipping

PFM converters with the high full-load efficiencies of

current-mode pulse-width modulation (PWM) convert-

ers. This control scheme allows the devices to achieve

high efficiencies over a wide range of loads, while the

current-sense function and high operating frequency

allow the use of miniature external components.

As with traditional PFM converters, the internal power

MOSFET is turned on when the voltage comparator

senses that the output is out of regulation (Figure 1).

However, unlike traditional PFM converters, switching is

accomplished through the combination of a peak cur-

rent limit and a pair of one-shots that set the maximum

on-time (16µs) and minimum off-time (2.3µs) for the

switch. Once off, the minimum off-time one-shot holds

the switch off for 2.3µs. After this minimum time, the

switch either 1) stays off if the output is in regulation, or

2) turns on again if the output is out of regulation.

The MAX764/MAX765/MAX766 limit the peak inductor

current, which allows them to run in continuous-con-

duction mode and maintain high efficiency with heavy

loads. (See the photo Continuous Conduction at Full

Current Limit in the

Typical Operating Characteristics.)

This current-limiting feature is a key component of the

control circuitry. Once turned on, the switch stays on

until either 1) the maximum on-time one shot turns it off

(16µs later), or 2) the current limit is reached.

To increase light-load efficiency, the current limit is set to

half the peak current limit for the first two pulses. If those

pulses bring the output voltage into regulation, the volt-

age comparator holds the MOSFET off and the current

limit remains at half the peak current limit. If the output

voltage is still out of regulation after two pulses, the cur-

rent limit is raised to its 0.75A peak for the next pulse.

(See the photo Discontinuous Conduction at Half and Full

Current Limit in the

Typical Operating Characteristics.)

Shutdown Mode

When SHDN is high, the MAX764/MAX765/MAX766

enter a shutdown mode in which the supply current

drops to less than 5µA. In this mode, the internal biasing

circuitry (including the reference) is turned off and OUT

discharges to ground. SHDN is a TTL/CMOS-logic level

input. Connect SHDN to GND for normal operation.

With a current-limited supply, power-up the device while

unloaded or in shutdown mode (hold SHDN high until V+

exceeds 3.0V) to save power and reduce power-up cur-

rent surges. (See the Supply Current vs. Supply Voltage

graph in the

Typical Operating Characteristics.)

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

_______________________________________________________________________________________

9

MAX764

MAX765

MAX766

SHDN

REF

LX

V+

GND

FB

V+

OUT

C4

68

µF

20V

D1

1N5817

L1

47

µH

8

6

7

3

4

2

1

C3

0.1

µF

C2

0.1

µF

C1

120

µF

20V

MAX764

MAX765

MAX766

-5

-12

-15

3 to 15

3 to 8

3 to 5

PRODUCT

OUTPUT

VOLTAGE (V)

INPUT

VOLTAGE (V)

5

Figure 2. Fixed Output Voltage Operation

MAX764

MAX765

MAX766

SHDN

REF

LX

V+

GND

FB

-1V to

-16V

V+

OUT

C4

68

µF

20V

D1

1N5817

L1

47

µH

8

6

7

3

4

2

1

C3

0.1

µF

C2

0.1

µF

C1

120

µF

20V

5

R2

R1

Figure 3. Adjustable Output Voltage Operation