LM5035B

SNVS613C – JULY 2009 – REVISED APRIL 2013

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The UVLO pin can also be used to implement various remote enable / disable functions. Turning off a converter

by forcing the UVLO pin to the standby condition provides a controlled soft-stop. See the Soft Start section for

more details.

Line Over Voltage / Load Over Voltage / Remote Thermal Protection

The LM5035B provides a multi-purpose OVP pin that supports several fault protection functions. When the OVP

pin voltage exceeds 1.25V, the controller is held in standby mode which immediately halts the PWM pulses at

the HO and LO pins. In standby mode, the VCC and REF bias regulators are active while the controller outputs

are disabled. When the OVP pin voltage falls below the 1.25V OVP threshold, the outputs are enabled and

normal soft-start sequence begins. Hysteresis is accomplished with an internal 23 µA current source that is

switched on or off into the impedance of the OVP pin set-point divider. When the OVP threshold is exceeded, the

current source is enabled to quickly raise the voltage at the OVP pin. When the OVP pin voltage falls below the

1.25V threshold, the current source is disabled causing the voltage at the OVP pin to quickly fall.

Several examples of the use of this pin are provided in the Application Information section.

Reference

The REF pin is the output of a 5V linear regulator that can be used to bias an opto-coupler transistor and

external housekeeping circuits. The regulator output is internally current limited to 15mA (minimum).

Cycle-by-Cycle Current Limit

The CS pin is driven by a signal representative of the transformer 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 32

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 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 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 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 LM5035B 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

LM5035B either sources or sinks current from the RES pin capacitor. If no current limit is detected during a cycle,

a 12 µA discharge current sink is enabled to pull the RES pin to ground. If a current limit is detected, the 12 µA

sink current is disabled and a 22µA current source causes the voltage at the RES pin to gradually increase. The

LM5035B 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 14):

•

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

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