SP6654 Datasheet, PDF (11/17 Pages) Sipex Corporation – High Efficiency 800mA Synchronous Buck Regulator Ideal for portable designs powered with Li Ion battery

OPERATION: Continued
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 K_ON/L.
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 reestablish itself. Power conversion
begins with the assertion of the internal refer-
ence ready signal which occurs approximately
150µs after the enable signal is received.
Power Good Indicator
A power good indicator looks at the voltage on
the feedback node. When this voltage is below
0.75V, the open drain NMOS on pin 3 sinks
current to ground. Tying a resistor from pin 3 to
V_IN or V_OUT creates a logic level power good
indicator. PWR_GD 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 SP6654
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
V_OUT (pin7). See the applications section for
resistor selection information.
APPLICATION INFORMATION
Inductor Selection
The SP6654 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, I_LR ≈ ^KON
L
For the typical SP6654 application circuit with
inductor size of 10µH, and K_ON of 2V*µsec, the
SP6654 current ripple would be about 200mA, and
would be fairly constant for different input and
output voltages, simplifying the selection of com-
ponents for the SP6654 power circuit. Other
inductor values could be selected, as shown in
Table 2 Components Selection. Using a larger
value than 10µH 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 SP6654, which has
a peak inductor current of 1.25A, most 15µH or
22µH inductors would have to be larger physi-
cal sizes, limiting their use in small portable
applications. Smaller values like 6.8µH would
more easily meet the 1.25A limit and come in
small case sizes, and the increased inductor
Date: 2/1/05
SP6654 High Efficiency 800mA Synchronous Buck Regulator
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