MAX1761
Small, Dual, High-Efficiency
Buck Controller for Notebooks
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The maximum on-time and minimum off-time, t
OFF(MIN)
,
one-shots restrict the continuous-conduction output
voltage. The worst-case dropout performance occurs
with the minimum on-time and the maximum off-time, so
the worst-case duty cycle for V
IN
= 6V, V
OUT1
= 5V is
given by:
The duty cycle is ideally determined by the ratio of
input-to-output voltage (Duty Cycle = V
OUT
/V
IN
).
Voltage losses in the loop cause the actual duty cycle
to deviate from this relationship. See the Dropout
Performance section for more information. Equate the
off-time duty cycle restriction to the nonideal input/out-
put voltage duty cycle ratio. Typical units will exhibit
better performance. Operation of any power supply in
dropout will greatly reduce the circuit’s transient
response, and some additional bulk capacitance may
be required to support fast load changes.
Resistive voltage drops in the inductor loop and the
dead-time effect cause switching-frequency variations.
Parasitic voltage losses decrease the effective voltage
applied to the inductor. The MAX1761 compensates by
shifting the duty cycle to maintain the regulated output
voltage. The resulting change in frequency is:
V
DROP1
is the sum of the parasitic voltage drops in the
inductor discharge path, including synchronous rectifi-
er, inductor, and PC board resistances; V
DROP2
is the
sum of the resistances in the charging path; and t
ON
is
the on-time calculated by the MAX1761.
In forced PWM mode, the dead-time effect increases
the effective on-time, reducing the switching frequency
as one or both dead times. This occurs only at light or
negative loads when the inductor current reverses.
Under these conditions, the inductor’s EMF causes the
switching node of the inductor to go high during the
dead time, extending the effective on-time.
.
.
