MIC2103 datasheet(21/38 Pages) MICREL

21 / 38 page

Micrel, Inc.

MIC2103/04

November 26, 2013

Revision 2.0

Figure 3b. MIC2103 Control Loop Timing

(Discontinuous Mode)

During discontinuous mode, the bias current of most

circuits are reduced. As a result, the total power supply

current during discontinuous mode is only about 400

μA,

allowing the MIC2103 to achieve high efficiency in light

load applications.

Soft-Start

Soft-start reduces the power supply input 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 MIC2103/04 implements an internal digital soft-start

by making the 0.8V reference voltage VREF ramp from 0

to 100% in about 6ms 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

circuitry

disabled

reduce

current

consumption. VDD must be powered up at the same time

or after VIN to make the soft-start function correctly.

Current Limit

The MIC2103/04 uses the RDS(ON) and external resistor

connected from ILIM pin to SW node to decide the

current limit.

Figure 4. MIC2103/04 Current Limiting Circuit

In each switching cycle of the MIC2103/04 converter, 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 RCL (VCL) is compared with the drop over the

bottom FET generating the short current limit. The small

capacitor (CCL) 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 RCL and CCL should be much less than the

minimum off time.

The VCL drop allows programming of short limit through

the value of the resistor (RCL), 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.

The short circuit current limit can be programmed by

using the following formula:

)

(

CLIM

)

(

∆

−

Eq. 3

where ISH = Desired Current limit

PP = Inductor current peak to peak

RDS (ON) = On resistance of low-side power MOSFET

VCL = Current limit threshold, the typical value is 14mV in

EC table

ICL = Current Limit source current, the typical value is

80µA in EC table.

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