MIC2104 datasheet(24/42 Pages) MICROCHIP

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MIC2103/4

DS20005899A-page 24

 2017 Microchip Technology Inc.

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.

4.3

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 MIC2103/4 implements an internal digital soft-start

by making the 0.8V reference voltage VREF ramp from

0 to 100% in about 6 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. VDD must be powered up at the same

time or after VIN to make the soft-start function

correctly.

4.4

Current-Limit

The MIC2103/4 uses the RDS(ON) and external resistor

connected from ILIM pin to SW node to decide the

current limit.

FIGURE 4-4:

MIC2103/4 Current Limiting

Circuit.

In each switching cycle of the MIC2103/4 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 150 ns. In this way the drop voltage

over the resistor RCL (VRCL) is compared with the drop

over the bottom FET generating the short current-limit.

The small capacitor (CCL) 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 RCL and CCL should be much

less than the minimum off time.

The VRCL 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 VRCL 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:

EQUATION 4-3:

In case of a hard short, the short limit is folded down to

allow an indefinite hard short on the output without any

destructive effect. It is mandatory to make sure that the

inductor current used to charge the output capacitance

during soft start is under the folded short limit,

otherwise the supply will go in hiccup mode and may

not be finishing the soft-start successfully.

The MOSFET RDS(ON) varies 30% to 40% with

temperature; therefore, it is recommended to add a

50% margin to the calculated RCL in Equation 4-3 to

avoid false current limiting due to increased MOSFET

junction temperature rise. It is also recommended to

connect SW pin directly to the drain of the low-side

MOSFET to accurately sense the MOSFETs RDS(ON).

4.5

MOSFET Gate Drive

The MIC2103/4 high-side drive circuit is designed to

switch an N-channel MOSFET. The Functional Block

Diagram shows a bootstrap circuit, consisting of D1 (a

Schottky diode is recommended) and CBST. This circuit

supplies energy to the high-side drive circuit. Capacitor

CBST is charged while the low-side MOSFET is on and

the voltage on the SW pin is approximately 0V. When

the high-side MOSFET driver is turned on, energy from

CBST is used to turn the MOSFET on. As the high-side

MOSFET turns on, the voltage on the SW pin increases

to approximately VIN. Diode D1 is reverse biased and

CBST floats high while continuing to keep the high-side

MOSFET on. The bias current of the high-side driver is

less than 10 mA, so a 0.1 μF to 1 μF is sufficient to hold

DETECTION

COUT

CIN

VIN

PGND

RCL

ILIM

CCL

CONTROL

LOGIC

TIMER

SOFT-START

ICL

CLIM

LPP





+0.5



 R

DS ON





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

Where:

ICLIM = Desired output current limit.

∆IL(PP) = Inductor current, peak-to-peak.

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

MOSFET.

VCL = Current-limit threshold. Typical value is 14 mV.

ICL = Current-limit source current. Typical value is

80 µA.

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