LM5035ASQ datasheet(14/30 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 ex-

ceeds 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 de-

pending 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 ex-

ternal 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 re-

duces 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 critical

for the current sense filter and sense resistor. The capacitor

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 trans-

former 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 sen-

sitive low power ground connections 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 LM5035A 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 LM5035A either sources or

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

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

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

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

source causes the voltage at the RES pin to gradually in-

crease. The LM5035A protects the converter with cycle-by-

cycle current limiting while the voltage at RES pin increases.

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 discharged

•

The soft-start current source is reduced from 110 µA to 1

µA

•

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 110 µA 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 12 µA current sink

and normal operation resumes

The overload timer function is very versatile and can be con-

figured for the following modes of protection:

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

disabled by connecting a zero to 50 k

Ω resistor from the RES

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

tion 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 overload

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 be-

fore initiating a hiccup mode restart, as previously described.

The dual advantages of this configuration are that a short term

overload will not cause a hiccup mode restart but during ex-

tended overload conditions, the average dissipation 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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