MIC45116 datasheet(21/42 Pages) MICROCHIP

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DS20005571A-page 21

MIC45116

FIGURE 4-4:

MIC45116-1 Control Loop

(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 350 µA, allowing the MIC45116-1 to achieve

high efficiency in light load applications.

4.3

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.

The MIC45116 implements an internal digital soft-start

by making the 0.8V reference voltage VREF ramp from

0 to 100% in about 3 ms with 9.7 mV 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.

4.4

Current Limit

The MIC45116 uses the RDS(ON) of the low-side

MOSFET and external resistor connected from the ILIM

pin to SW node to set the current limit.

In each switching cycle of the MIC45116, the inductor

current is sensed by monitoring the low-side MOSFET

in the OFF period. The sensed voltage VLIM is

compared with the power ground (PGND) after a

blanking time of 150 ns. In this way the drop voltage

over the resistor R26 (VCL) is compared with the drop

over the bottom FET generating the short current limit.

The small capacitor (C16) connected from the ILIM pin

to PGND filters the switching node ringing during the

off-time allowing a better short-limit measurement. The

time constant created by R26 and C16 should be much

less than the minimum off time.

FIGURE 4-5:

MIC45116 Current-Limiting

Circuit.

The VCL drop allows short-limit programming based on

the value of the resistor (R26). If the absolute value of

the voltage drop on the bottom FET becomes greater

than VCL, and the VILIM falls below PGND, an

overcurrent is triggered causing the IC to enter hiccup

mode. 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 Equation 4-3.

EQUATION 4-3:

ESTIMATED ON-TIME

L CROSSES 0 AND VFB > 0.8

DISCONTINUOUS MODE STARTS

REF

FB < 0.8. WAKE UP FROM

DISCONTINUOUS MODE

PVIN

MIC45116

BST

PGND

ILIM

R26

C16

VIN

R26

CLIM

LPP





+0.5



0.1

–

 R

DS ON





-------------------------------------------------------------------------------------------------------------------

Where:

ICLIM

Desired current limit.

RDS(ON)

On-resistance of low-side power

MOSFET, 16 mΩ typically.

VCL

Current-limit threshold (typical

absolute value is 14 mV).

ICL

Current-limit source current (typical

value is 80 µA).

∆IL(PP)

Inductor current peak-to-peak,

since the inductor is integrated, use

Equation 4-4 to calculate the

inductor ripple current.

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