MCP16501 datasheet(30/42 Pages) MICROCHIP

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MCP16501

DS20006388E-page 30

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5.5.1

OVERCURRENT PROTECTION

(BUCK CHANNELS)

The overcurrent protection consists of a cycle-by-cycle,

high-side current limit with digital filtering, followed by

the Hiccup mode for protection against short-circuit

conditions.

The cycle-by-cycle, high-side current limit includes

frequency

foldback.

Because

Leading-Edge

Blanking (LEB) in Peak Current mode control,

frequency foldback (with a factor = 4) is used to allow

more time for inductor discharge and prevent current

runaway in a deep overload condition.

Frequency foldback operation is entered when:

A high-side current limit event has been

detected; and

The feedback voltage is less than 500 mV

(typical).

Cycle-by-cycle overcurrent protection with frequency

foldback is always active and it is the first current limit

protection mechanism.

The second current limit mechanism is Hiccup mode

protection, which is also always enabled, including

during the soft start ramp.

Since the Hiccup mode protection is also active during

soft start, there will be a limitation on the maximum

simultaneous DC and capacitive loading to ensure that

the Hiccup mode protection will not be engaged during

the soft start ramp. This is further explained in

Section 5.6 “Maximum Simultaneous Capacitive

and DC Loading in Soft Start”.

Hiccup mode is invoked based on digital counting of

High-Side Overcurrent (HS OC) events, regardless of

the frequency at which they take place (full switching

frequency or foldback switching frequency).

Each time the overcurrent protection detects a

high-side current limit event, the current on-time is

terminated and a HS OC event counter is incremented.

The length of the counter is four bits.

If the counter reaches its End-of-Count (EOC) while the

instantaneous value of the POK signal is still low, the

Buck converter is turned off (both high-side and

low-side transistors are turned off) and Hiccup mode

protection is triggered.

The intervention of Hiccup mode on any of the Buck

channels can have two different behaviors, depending

on the HCPEN status of the channel affected by

overcurrent conditions. By default, all device options of

the MCP16501 come with HCPEN enabled, with the

exception of MCP16501D.

If HCPEN is disabled, the intervention of Hiccup mode

will immediately terminate the power delivery on all

channels, including LDO. At the same time, nRSTO will

be asserted low.

After a 100 ms delay, the MCP16501D will automati-

cally attempt a new start-up sequence without the need

of an external Start condition (from nSTRT or

PWRHLD).

If HCPEN is enabled, the intervention of Hiccup mode

only affects the responsible Buck channel. All other

channels will continue to operate normally. If the

affected channel is part of the power-up sequence,

nRSTO will be asserted low.

The affected channel will be kept off for a certain

Hiccup time (tHICCUP), which corresponds to 3x soft

start time on that channel, and after the Hiccup time, a

new soft start is attempted.

If the short-circuit condition is removed and the affected

channel resumes normal operation, nRSTO will be

asserted high after the Reset delay (t4).

The HS OC event counter is reset only after

15 consecutive HS turn-on pulses without any

overcurrent event. This counting is done by the Reset

counter.

Note that all counting is switching event-based, so it is

not relevant if the switching takes place at fsw or at fsw/4

(i.e., in frequency foldback).

The Hiccup mode flowchart is detailed in Figure 5-1.

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