Buck regulator handles light loads
If your design uses too small a boost capacitor, it will not operate reliably.
Justin Larson and Frank Kolanko, On Semiconductor, East Greenwich, RI; Edited by Martin Rowe and Fran Granville -- EDN, September 9, 2010
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Buck regulators operating in
CCM (continuous-conduction
mode) have straightforward operation,
allowing for easy calculation of output
voltage and system design. However,
lightly loaded buck regulators operate
in DCM (discontinuous-conduction
mode), and their operation is more
complicated. The duty cycle changes
from a ratio of the output voltage and
the input voltage. A regulator that reduces
a 12V input to 6V has a 50% duty
cycle. When the regulator is too lightly
loaded to keep some current continuously
flowing in the inductor, it enters
DCM. The duty cycle changes to a
complex function of inductor value,
input voltage, switching frequency, and
output current, greatly slowing the control-loop response.| Read More Design Ideas |
During CCM, current always flows
through the inductor. Q1 or D2 supplies
this current during the flyback
event that Q1’s turn-off causes (Figure
2). The flyback event creates a voltage
at the source of Q1, and the drop across
D2 limits this voltage, making it a negative
voltage with respect to ground. Sufficient
voltage is available to drive Q1
because the CBOOST capacitor boosts the
gate voltage. This boost provides a high voltage to the boost pin and the resultant
negative voltage on the Q1 source.The system enters DCM when the
load drops to the point at which the average
current demand is less than onehalf
the current ripple. Diode D2 prevents
reverse current in the inductor. Depending on the chip you use, the
output may overshoot due to the slower
response time of the control loop. The
regulator may also miss pulses and generally
operate unpredictably. After Q1
turns off, CBOOST starts to bleed down
through the boost pin and D1 (Figure
3). The extended off time of Q1 in DCM starts to discharge the CBOOST capacitor.
At approximately 3V across CBOOST, Q1
does not turn on until the output capacitor,
COUT, discharges adequately to provide
a lower voltage on the source of Q1
than that of the boost pin through D1.
This behavior is unacceptable in a voltage
regulator.
High temperatures create a situation
with higher leakage currents. You don’t
know the temperature coefficient of
the current into the boost pin, so you
should also check operation at low temperature.
Evaluate the system to determine
the lowest capacitor value, using
this result in your worst-case evaluation
simulations. You can thus ensure that
the design will operate in DCM by increasing
the value of CBOOST. You could
also increase the reference voltage to
which D1 connects. You may want to
consider replacing D1 with a low-leakage
Schottky diode. If none of these approaches
results in reliable operation,
you can switch to an IC that uses a gate
driver referenced to ground or modify
your design to use a synchronous-buck
architecture.
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