Datasheet

LM5035

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SNVS428G –JANUARY 2006–REVISED MARCH 2013

Line Over Voltage / Load Over Voltage / Remote Thermal Protection

The LM5035 provides a multipurpose 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 23µA 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 Applications Information.

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 20mA (typical).

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 32Ω 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).

Product Folder Links: LM5035