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Date: 7/9/04

SP6656 High Efficiency 400mA Synchronous Buck Regulator with Bit Programmable Output

© Copyright 2004 Sipex Corporation

For the typical SP6656 application circuit with

inductor size of 10

SP6656 current ripple would be about 200mA, and

mode and improves light load efficiency when

the load current is less than half the inductor

Thermal Shutdown

The converter will open both power switches if

the die junction temperature rises above 140

°C.

The die must cool down below 126

°C before the

regulator is re-enabled. This feature protects the

SP6656 and surrounding circuitry from exces-

sive power dissipation due to fault conditions.

Shutdown/Enable Control

The EN pins of the device are logic level control

pins that shut down the converter when a logic

is low, or enables the converter when a logic is

high. When the converter is shut down, the

power switches are opened and all circuit bias-

ing is extinguished leaving only junction leak-

age currents on supply pins 1 and 2. After pins

4 or 5 are brought high to enable the converter,

there is a turn on delay to allow the regulator

circuitry to re-establish itself. Power conversion

begins with the assertion of the internal refer-

ence ready signal which occurs approximately

150

µs after the enable signal is received.

Output Voltage Selection

The OVSI (Output Voltage Selection Input) pin

is internally driving a NMOS Gate which Drain

achieve a a higher output voltage ( see table 1 for

calculation, page 3)

External Feedback Pin

The FB pin 6 is compared to an internal refer-

ence voltage of 0.8V to regulate the SP6656

output. The output voltage can be externally

programmed within the range +1.0V to +5.0V

by tying a resistor from FB to ground and FB to

resistor selection information.

Inductor Selection

The SP6656 uses a specially adapted minimum

on-time control of regulation utilizing a preci-

sion comparator and bandgap reference. This

adaptive minimum on-time control has the ad-

vantage of setting a constant current ripple for a

given inductor size. From the operations section

it has been shown:

L

THEORY OF OPERATION

APPLICATION INFORMATION

would be fairly constant for different input and

output voltages, simplifying the selection of com-

ponents for the SP6656 power circuit. Other

inductor values could be selected, as shown in

Table 2 Components Selection. Using a larger

value than 10

µH in an attempt to reduce output

voltage ripple would reduce inductor current ripple

and may not produce as stable an output ripple.

For larger inductors with the SP6656, which has

a peak inductor current of 0.625A, most 15

µH

or 22

µH inductors would have to be larger

physical sizes, limiting their use in small por-

table applications. Smaller values like 10

µH

would more easily meet the 0.625A limit and

come in small case sizes, and the increased