MIC45116 datasheet(21/42 Pages) MICROCHIP

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2016 - 2022 Microchip Technology Inc.

DS20005571B-page 21

MIC45116

Finally, the high-side driver is turned on. Figure 4-4

shows the control loop timing in discontinuous mode.

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 VILIM is

compared with the power ground (PGND) after a

blanking time of 150 ns.

In this way, the drop voltage over the resistor R26

(VR26) 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 VR26 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 VR26, 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.

ESTIMATED ON-TIME

L CROSSES 0 AND VFB > 0.8V

DISCONTINUOUS MODE STARTS

REF

FB < 0.8V WAKE UP FROM

DISCONTINUOUS MODE

PVIN

MIC45116

BST

PGND

ILIM

R26

C16

VIN

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