MIC28304 datasheet(24/39 Pages) MICREL

24 / 39 page

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 400

μA, 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 VREF 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 VFB 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 VIN

to make the soft-start function correctly.

Current Limit

The MIC28304 uses the RDS(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 VCL 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 VCL. 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

Revision 1.1

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