MCP19035-AAAAESLASHMF datasheet(16/44 Pages) MICROCHIP

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MCP19035

DS22326B-page 16

 2012-2013 Microchip Technology Inc.

For the low-side overcurrent protection, when the

sensed voltage drop across the low-side MOSFET is

greater than the low-side overcurrent threshold voltage

specified,

low-side

overcurrent

counter

incremented by one count. On the next cycle, if the low-

side over current threshold voltage is not exceeded, the

low-side overcurrent counter is decreased by one. If

the low-side overcurrent counter reaches a count of 7,

a fault condition exists and the MCP19035 turns off

both external MOSFETs. After a 60 ms delay, the

MCP19035 device will attempt to restart. If during the

next cycle, a low-side overcurrent threshold voltage is

measured across the low-side MOSFET, a fault is

again declared and both external MOSFETs are turned

off for another 60 ms. However, if after the attempted

restart a low-side overcurrent threshold voltage is not

measured across the low-side MOSFET, the low-side

overcurrent counter is decreased by one and the

MCP19035 continues to operate until the low-side

overcurrent counter reaches a count of 7.

The voltage threshold for high-side overcurrent

protection circuit is fixed, 480 mV typical. The high-side

voltage threshold will also depend on the value of the

voltage across the bootstrap circuit capacitor, and will

decrease when this voltage decreases. This will ensure

that the high-side protection will avoid a failure of the

MOSFET when the bootstrap voltage is low and the

switching losses are high. This threshold will provide a

cycle-by-cycle protection in case of short circuit, but it

should not be used to provide a precise current limit for

the converter. An estimation of the current that flows in

the high-side MOSFET during short circuit can be

found using Equation 4-2. Note that, due to the leading

edge blanking time, this current also depends on the

inductor's ripple current. To avoid false triggering of the

high-side

overcurrent

protection

circuit

during

transients, it is highly recommended to choose a

MOSFET that will provide a threshold at least four

times higher than the maximum output current of the

converter.

EQUATION 4-2:

PEAK CURRENT FOR

HIGH-SIDE MOSFET

The voltage threshold for the low-side overcurrent

protection circuit is fixed, 180 mV typical. Different

values for this threshold (from 100 mV to 300 mV) are

available on request. An estimation of the current that

flows on the low-side MOSFET during short circuit is

realized using Equation 4-3. Note that, due to the

leading edge blanking time, this current also depends

on the inductor's ripple current. To avoid false triggering

of the low-side over current protection circuit during

transients, it is highly recommended to choose a

MOSFET that will provide a threshold at least two times

higher than the maximum output current of the

converter.

EQUATION 4-3:

To avoid a false trigger of the overcurrent circuit, a

leading edge blanking circuit is present on both the

high and low-side measurements. Due to this blanking

time, the accuracy of the overcurrent circuit may be

impacted if the converter operates at higher duty cycles

(more than 85%), or if the inductor's current ripple is

very high (i.e. the inductor is saturated by the excessive

current).

HS MOS

OC HS

DSON

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

Where:

IHS MOS = Current that passes through the

High-Side MOSFET

VOC HS = Threshold Voltage for High-Side

Overcurrent Protection Circuit

(480 mV)

RDSON = ON Resistance of the High-Side

MOSFET

LS MOS

OC LS

DSON

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

Where:

ILS MOS = Current that passes through the

Low-Side MOSFET

VOC LS = Threshold Voltage for Low-Side

Overcurrent Protection Circuit

(180 mV)

RDSON = ON Resistance of the Low-Side

MOSFET

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