MIC28304 Datasheet, PDF (24/39 Pages) Micrel Semiconductor – 70V 3A Power Module

Micrel, Inc.
MIC28304
Figure 3. MIC28302-1 Control Loop Timing
(Discontinuous Mode)
During discontinuous mode, the bias current of most
circuits is substantially reduced. As a result, the total
power supply current during discontinuous mode is only
about 400μA, allowing the MIC28304-1 to achieve high
efficiency in light load applications.
Soft-Start
Soft-start reduces the input power supply surge current at
startup by controlling the output voltage rise time. The
input surge appears while the output capacitor is charged
up. A slower output rise time will draw a lower input surge
current.
The MIC28304 implements an internal digital soft-start by
making the 0.8V reference voltage V_REF ramp from 0 to
100% in about 5ms with 9.7mV steps. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V_FB ramp. Once the soft-start cycle ends, the related
circuitry is disabled to reduce current consumption.
PVDD must be powered up at the same time or after V_IN
to make the soft-start function correctly.
Current Limit
The MIC28304 uses the R_DS(ON) of the low side
MOSEFET and external resistor connected from ILIM pin
to SW node to decide the current limit.
Figure 4. MIC28304 Current-Limiting Circuit
In each switching cycle of the MIC28304, the inductor
current is sensed by monitoring the low-side MOSFET in
the OFF period. The sensed voltage V(ILIM) is compared
with the power ground (PGND) after a blanking time of
150ns. In this way the drop voltage over the resistor R15
(VCL) is compared with the drop over the bottom FET
generating the short current limit. The small capacitor
(C6) connected from ILIM pin to PGND filters the
switching node ringing during the off-time allowing a
better short limit measurement. The time constant
created by R15 and C6 should be much less than the
minimum off time.
The V_CL drop allows programming of short limit through
the value of the resistor (R15), If the absolute value of the
voltage drop on the bottom FET is greater than V_CL. In
that case the V(ILIM) is lower than PGND and a short
circuit event is triggered. A hiccup cycle to treat the short
event is generated. The hiccup sequence including the
soft start reduces the stress on the switching FETs and
protects the load and supply for severe short conditions.
March 25, 2014
24
Revision 1.1