MIC45205 datasheet(16/40 Pages) MICROCHIP

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MIC45205

DS20005798B-page 16



2017 - 2022 Microchip Technology Inc.

FIGURE 4-4:

MIC45205-1 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 350 µA, allowing the MIC45205-1 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 MIC45205 implements an internal digital soft-start

by making the 0.8V reference voltage VREF ramp from

0 to 100% in about 5 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 MIC45205 uses the RDS(ON) of the low-side

MOSFET and external resistor connected from ILIM pin

to SW node to set the current limit.

FIGURE 4-5:

MIC45205 Current-Limiting

Circuit.

In each switching cycle of the MIC45205, 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 R15 (VR15) is compared with the drop

over the bottom FET generating the short current limit.

The small capacitor (C15) 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 C15 should be much

less than the minimum off time.

The VR15 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 becomes

greater than VR15, 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:

L CROSSES 0 AND VFB > 0.8V

DISCONTINUOUS MODE STARTS

FB < 0.8V WAKE-UP FROM

DISCONTINUOUS MODE

REF

ESTIMATED ON-TIME

VIN

MIC45205

BST

PGND

ILIM

R15

C15

R15

ICLIM

IL PP







0.5







RDS ON





VCL_OFFSET



ICL

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

Where:

ICLIM

Desired current limit.

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

MOSFET, 16 mΩ typically.

VCL_

OFFSET

Current-limit threshold offset (typ.

14 mV).

ICL

Current-limit source current (typ.

70 µA).

ΔIL(PP)

Inductor current peak-to-peak.

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