MIC22205 Datasheet, PDF (18/29 Pages) Micrel Semiconductor – 2A, Integrated, Switch, High-Efficiency

Micrel, Inc.
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 percent of the reference voltage, the low-side
MOSFET will begin switching.
MIC22205
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.
Figure 4. EN Turn-On at 1V Pre-Bias
When the MIC22205 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
2A load pulls the output down fast enough to force the
high-side MOSFET on (V_SW trace).
Figure 6. EN Turn-Off − 200mA Load
Current Limit
The MIC22205 uses a two-stage technique to protect
against overload. The first stage is to limit the current in
the P-channel switch; the second is over temperature
shutdown.
Current is limited by measuring the current through the
high-side MOSFET during its power stroke and
immediately switching off the driver when the preset limit
is exceeded.
The circuit in Figure 7 describes the operation of the
current limit circuit. Since the actual RDS_ON of the P-
channel MOSFET varies from part to part and with
changes in temperature and input voltage, simple IR
voltage detection is not employed. Instead, a smaller
copy of the Power MOSFET (Reference FET) is fed with
a constant current which is a directly proportional to the
factory set current limit. This sets the current limit as a
current ratio and thus, is not dependant upon the RDS_ON
value. Current limit is set to nominal value. Variations in
the scale factor K between the power PFET and the
reference PFET used to generate the limit threshold
account for a relatively small inaccuracy.
Figure 5. EN Turn-OFF − 2A Load
August 2011
18
M9999-082511-A