LM5035 datasheet(12/28 Pages) NSC

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Cycle-by-Cycle Current Limit

The CS pin is driven by a signal representative of the trans-

former primary current. If the voltage sensed at CS pin

exceeds 0.25V, the current sense comparator terminates the

HO or LO output driver pulse. If the high current condition

persists, the controller operates in a cycle-by-cycle current

limit mode with duty cycle determined by the current sense

comparator instead of the PWM comparator. Cycle-by-cycle

current limiting may trigger the hiccup mode restart cycle

depending on the configuration of the RES pin (see below).

A small R-C filter connect to the CS pin and located near the

controller is recommended to suppress noise. An internal

Ω MOSFET connected to the CS input discharges the

external current sense filter capacitor at the conclusion of

every cycle. The discharge MOSFET remains on for an

additional 50 ns after the HO or LO driver switches high to

blank leading edge transients in the current sensing circuit.

Discharging the CS pin filter each cycle and blanking leading

edge spikes reduces the filtering requirements and improves

the current sense response time.

The current sense comparator is very fast and responds to

short duration noise pulses. Layout considerations are criti-

cal for the current sense filter and sense resistor. The ca-

pacitor associated with the CS filter must be placed very

close to the device and connected directly to the CS and

AGND pins. If a current sense transformer is used, both

leads of the transformer secondary should be routed to the

filter network, which should be located close to the IC. If a

sense resistor located in the source of the main MOSFET

switch is used for current sensing, a low inductance type of

resistor is required. When designing with a current sense

resistor, all of the noise sensitive low power ground connec-

tions should be connected together near the AGND pin, and

a single connection should be made to the power ground

(sense resistor ground point).

Overload Protection Timer

The LM5035 provides a current limit restart timer to disable

the outputs and force a delayed restart (hiccup mode) if a

current limit condition is repeatedly sensed. The number of

cycle-by-cycle current limit events required to trigger the

restart is programmable by the external capacitor at the RES

pin. During each PWM cycle, the LM5035 either sources or

sinks current from the RES pin capacitor. If no current limit is

detected during a cycle, an 12µA discharge current sink is

enabled to pull the RES pin to ground. If a current limit is

detected, the 12µA sink current is disabled and a 22µA

current source causes the voltage at the RES pin to gradu-

ally increase. The LM5035 protects the converter with cycle-

by-cycle current limiting while the voltage at RES pin in-

creases. If the RES voltage reaches the 2.5V threshold, the

following restart sequence occurs (also see Figure 2):

•

The RES capacitor and SS capacitors are fully dis-

charged

•

The soft-start current source is reduced from 55µA to 1µA

•

The SS capacitor voltage slowly increases. When the SS

voltage reaches )1V, the PWM comparator will produce

the first narrow output pulse. After the first pulse occurs,

the SS source current reverts to the normal 55µA level.

The SS voltage increases at its normal rate, gradually

increasing the duty cycle of the output drivers

•

If the overload condition persists after restart, cycle-by-

cycle current limiting will begin to increase the voltage on

the RES capacitor again, repeating the hiccup mode

sequence

•

If the overload condition no longer exists after restart, the

RES pin will be held at ground by the 12µA current sink

and normal operation resumes

The overload timer function is very versatile and can be

configured for the following modes of protection:

1. Cycle-by-cycle only: The hiccup mode can be com-

pletely disabled by connecting a zero to 50k

Ω resistor from

the RES pin to AGND. In this configuration, the cycle-by-

cycle protection will limit the output current indefinitely and

no hiccup sequences will occur.

2. Hiccup only: The timer can be configured for immediate

activation of a hiccup sequence upon detection of an over-

load by leaving the RES pin open circuit.

3. Delayed Hiccup: Connecting a capacitor to the RES pin

provides a programmed interval of cycle-by-cycle limiting

before initiating a hiccup mode restart, as previously de-

scribed. The dual advantages of this configuration are that a

short term overload will not cause a hiccup mode restart but

during extended overload conditions, the average dissipa-

tion of the power converter will be very low.

4. Externally Controlled Hiccup: The RES pin can also be

used as an input. By externally driving the pin to a level

greater than the 2.5V hiccup threshold, the controller will be

forced into the delayed restart sequence. For example, the

external trigger for a delayed restart sequence could come

from an over-temperature protection circuit or an output

over-voltage sensor.

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