MIC22705_11 Datasheet, PDF (17/29 Pages) Micrel Semiconductor – 1MHz, 7A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Pre-Bias Start-Up
The MIC22705 is designed to start-up into a pre-biased
output. This prevents large negative inductor currents
and excessive output voltage oscillations. The
MIC22705 starts with the low-side MOSFET turned off,
preventing reverse inductor current flow. The
synchronous MOSFET stays off until the Power Good
(PG) goes high after the V_FB is above 90% of V_REF.
A pre-bias condition can occur if the input is turned off
then immediately turned back on before the output
capacitor is discharged to ground. It is also possible that
the output of the MIC22705 could be pulled up or pre-
biased through parasitic conduction paths from one
supply rail to another in multiple voltage (V_OUT) level ICs
such as a FPGA.
Figure 3 shows a normal start-up waveform. A 1µA
current source charges the soft-start capacitor C_RC. The
C_RC capacitor forces the V_RC voltage to come up slowly
(V_RC trace), thereby providing a soft-start ramp. This
ramp is used to control the internal reference (V_REF).
The error amplifier forces the output voltage to follow the
V_REF ramp from zero to the final value.
MIC22705
Figure 4. EN Turn-On at 1V Pre-Bias
When the MIC22705 is turned off, the low-side MOSFET
will be disabled and the output voltage will decay to zero.
During this time, the ramp control voltage (V_RC) will still
control the output voltage fall-time with the high-side
MOSFET if the output voltage falls faster than the V_RC
voltage. Figure 5 shows this operating condition. Here a
7A load pulls the output down fast enough to force the
high-side MOSFET on (V_SW trace).
Figure 3. EN Turn-On Time − Normal Start-Up
If the output is pre-biased to a voltage above the
expected value, as shown in Figure 4, then neither
MOSFET will turn on until the ramp control voltage (V_RC)
is above the reference voltage (V_REF). Then, the high-
side MOSFET starts switching, forcing the output to
follow the V_RC ramp. Once the feedback voltage is
above 90% of the reference voltage, the low-side
MOSFET will begin switching.
Figure 5. EN Turn-OFF − 7A Load
If the output voltage falls slower than the V_RC voltage,
then the both MOSFETs will be off and the output will
decay to zero as shown in the V_OUT trace in Figure 6.
With both MOSFETs off, any resistive load connected to
the output will help pull down the output voltage. This
will occur at a rate determined by the resistance of the
load and the output capacitance.
March 2011
17
M9999-033111-A