SP6655ER-L Datasheet, PDF (11/16 Pages) Sipex Corporation – High Efficiency 400mA Synchronous Buck Regulator

THEORY OF OPERATION
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 K_ON/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
150µs after the enable signal is received.
Battery Low Indicator
The BLON function is a differential measure-
ment of (V_IN -V_OUT) which causes the open
drain NMOS on pin 3 to sink current to ground
when (V_IN -V_OUT) < 300mV. Tying a resistor
from pin 3 to V_IN or V_OUT 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
V_OUT (pin7). See the applications section for
resistor selection information.
APPLICATION 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, I_LR ≈ ^KON
L
For the typical SP6655 application circuit with
inductor size of 10µH, and K_ON of 2V*µsec, the
SP6655 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 SP6655 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 SP6655, which has
a peak inductor current of 0.625A, most 15µH
or 22µH inductors would have to be larger
physical sizes, limiting their use in small por-
table applications. Smaller values like 10µH
would more easily meet the 0.625A limit and
come in small case sizes, and the increased
Date: 09/27/05
SP6655 High Efficiency 400mA Synchronous Buck Regulator
11
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