MIC2127AYML-T5 datasheet(21/38 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005676B-page 21

MIC2127A

EQUATION 4-4:

Since MOSFET RDS(ON) varies from 30%-40% with

temperature, it is recommended to consider the

RDS(ON) variation while calculating RCL in the above

equation, to avoid false current limiting due to

increased MOSFET junction temperature rise. Also

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

MOSFET to accurately sense the MOSFETs RDS(ON).

To improve the current limit variation, the MIC2127A

adjusts the internal source current of the current limit

(ICL) at a rate of 0.3 µA/°C when the MIC2127A

junction temperature changes to compensate the

RDS(ON) variation of external low-side MOSFET. The

effectiveness of this method depends on the thermal

gradient between the MIC2127A and the external

low-side MOSFET. The lower the thermal gradient, the

better the current limit variation.

A small capacitor (CCL) can be connected from the ILIM

pin to PGND to filter the switch node ringing during the

OFF time, allowing a better current sensing. The time

constant of RCL and CCL should be less than the

minimum OFF time.

4.4

Negative Current Limit

The MIC2127A implements negative current limit by

sensing the SW voltage when the low-side FET is ON.

If the SW node voltage exceeds 48 mV typical, the

device turns off the low-side FET for 500 ns. Negative

current limit value is shown in Equation 4-5.

EQUATION 4-5:

4.5

High-Side MOSFET Gate Drive

(DH)

The MIC2127A's high-side drive circuit is designed to

switch

N-Channel

external

MOSFET.

The

MIC2127A Functional Block diagram shows a

bootstrap diode between the PVDD and BST pins. This

circuit supplies energy to the high-side drive circuit. A

low ESR ceramic capacitor should be connected

between BST and SW pins (refer to the Typical

Application circuit).The capacitor between BST and

SW pins, CBST, is charged while the low-side MOSFET

is on. When the high-side MOSFET driver is turned on,

energy from CBST is used to turn the MOSFET on. A

minimum of 0.1 µF low ESR ceramic capacitor is

recommended between BST and SW pins. The

required value of CBST can be calculated using the

following Equation 4-6:

EQUATION 4-6:

A small resistor in series with CBST can be used to slow

down the turn-on time of the high-side N-channel

MOSFET.

4.6

Low-Side MOSFET Gate Drive (DL)

The MIC2127A's low-side drive circuit is designed to

switch an N-Channel external MOSFET. The internal

low-side MOSFET driver is powered by PVDD. Connect

a minimum of 4.7 µF low-ESR ceramic capacitor to

supply the transient gate current of the external

MOSFET.

4.7

Auxiliary Bootstrap LDO

(EXTVDD)

The MIC2127A features an auxiliary bootstrap LDO

that improves the system efficiency by supplying the

MIC2127A internal circuit bias power and gate drivers

from the converter output voltage. This LDO is enabled

when the voltage on the EXTVDD pin is above 4.6V

(typical) and, at the same time, the main LDO that

operates from VIN is disabled to reduce power

consumption.

CLIM

IL

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





DS ON



OFFSET



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

Where:

ICLIM

= Load current limit

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

ILPP

= Inductor peak-to-peak ripple current

VOFFSET = Current-limit comparator offset (15 mV max.)

ICL

= Current-limit source current (100 µA typ)

NLIM

48mV

DS ON



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

Where:

INLIM

= Negative current limit

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

BST

G_HS

V

BST

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

Where:

QG_HS

= High-side MOSFET total gate charge

VBST

= Drop across the CBST,

generally 50 mV to 100 mV

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