MIC2169B Datasheet, PDF (16/24 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

Micrel, Inc.
MIC2169B
Figure 9. Open-Loop Gain Margin
Figure 11. MIC2169A Startup without Pre-Bias Support
Figure 11 shows MIC2169A startup with a pre-bias of 1V
on the output, in which the pre-existing output voltage
discharges during soft start.
Figure 10. Open-Loop Phase Margin
Pre-Biased Loads
The MIC2169B supports pre-biased loads. Some
applications have a pre-existing voltage on the output.
This pre-existing or pre-biased load is generated by an
external supply (other than the MIC2169B). During
startup without pre-bias support, MIC2169A will pull the
output voltage to ground through the inductor and low
side FET (see figure 11).
The MIC2169B prevents the current sinking of any pre-
existing voltage source at the output (see figure 12). It
does this by keeping the low side FET off during the soft
start period. In some applications this pre-bias current
sink is not a problem, and the MIC2169A may be used.
In some applications the pre-bias current sink may
cause a problem, and the MIC2169B should be used.
The MIC2169B can support up to 90% of a pre-bias
condition (up to 90% of the final regulated output
voltage) see figure 13.
Figure 12. MIC2169B startup with pre-bias support
Figure 12 shows MIC2169B startup with a pre-bias of 1V
on the output, in which the pre-existing output voltage
has no discharge.
June 2009
16
M9999- 060309-A
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