SP6655 datasheet(11/16 Pages) SIPEX

11 / 16 page

Date: 09/27/05

SP6655 High Efficiency 400mA Synchronous Buck Regulator

For the typical SP6655 application circuit with

inductor size of 10

µH, and KON of 2V*µsec, the

SP6655 current ripple would be about 200mA, and

driver latch and open the switch to prevent

inductor current reversal. This circuit along

with the on-timer puts the converter into PFM

mode and improves light load efficiency when

the load current is less than half the inductor

ripple current defined by KON/L.

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

SP6655 and surrounding circuitry from exces-

sive power dissipation due to fault conditions.

Shutdown/Enable Control

The D0, D1 pins 4,5 of the device are logic level

control pins that according to Table 1 shut down

the converter when both are a logic low, or

enables the converter when either are a logic

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.

Battery Low Indicator

The BLON function is a differential measure-

ment of (VIN -VOUT) which causes the open

drain NMOS on pin 3 to sink current to ground

when (VIN -VOUT) < 300mV. Tying a resistor

from pin 3 to VIN or VOUT creates a logic level

battery low indicator. A low bandwidth com-

parator and 3% hysteresis filter the input voltage

ripple to prevent noisy transitions at the thresh

old. BLON is forced Low when in UVLO.

External Feedback Pin

The FB pin 6 is compared to an internal refer-

ence voltage of 0.8V to regulate the SP6655

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

VOUT (pin7). See the applications section for

resistor selection information.

Inductor Selection

The SP6655 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:

Inductor Current Ripple, ILR ≈

KON

THEORY OF OPERATION

APPLICATION INFORMATION

would be fairly constant for different input and

output voltages, simplifying the selection of com-

ponents for the SP6655 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

voltageripplewouldreduceinductorcurrentripple

and may not produce as stable an output ripple.

For larger inductors with the SP6655, 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

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