LM5035 datasheet(11/28 Pages) NSC

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Functional Description

The LM5035 PWM controller contains all of the features

necessary to implement half-bridge voltage-mode controlled

power converters. The LM5035 provides two gate driver

outputs to directly drive the primary side power MOSFETs

and two signal level outputs to control secondary synchro-

nous rectifiers through an isolation interface. Secondary side

drivers, such as the LM5110, are typically used to provide

the necessary gate drive current to control the sync MOS-

FETs. Synchronous rectification allows higher conversion

efficiency and greater power density than conventional PN or

Schottky rectifier techniques. The LM5035 can be configured

to operate with bias voltages ranging from 8V to 105V.

Additional features include line under-voltage lockout, cycle-

by-cycle current limit, voltage feed-forward compensation,

hiccup mode fault protection with adjustable delays, soft-

start, a 2MHz capable oscillator with synchronization capa-

bility, precision reference, thermal shutdown and program-

mable volt

•second clamping. These features simplify the

design of voltage-mode half-bridge DC-DC power convert-

ers. The Functional Block Diagram is shown in Figure 1.

High-Voltage Start-up Regulator

The LM5035 contains an internal high voltage start-up regu-

lator that allows the input pin (VIN) to be connected directly

to a nominal 48 VDC input voltage. The regulator input can

withstand transients up to 105V. The regulator output at VCC

(7.6V) is internally current limited to 25 mA typical. When the

UVLO pin potential is greater than 0.4V, the VCC regulator is

enabled to charge an external capacitor connected to the

VCC pin. The VCC regulator provides power to the voltage

reference (REF) and the output drivers (LO, SR1 and SR2).

When the voltage on the VCC pin exceeds the UVLO thresh-

old of 7.6V, the internal voltage reference (REF) reaches its

regulation setpoint of 5V and the UVLO voltage is greater

than 1.25V, the controller outputs are enabled. The value of

the VCC capacitor depends on the total system design, and

its start-up characteristics. The recommended range of val-

ues for the VCC capacitor is 0.1µF to 100µF.

The VCC under-voltage comparator threshold is lowered to

6.2V (typical) after VCC reaches the regulation set-point. If

VCC falls below this value, the outputs are disabled, and the

soft-start capacitor is discharged. If VCC increases above

7.6V, the outputs will be enabled and a soft-start sequence

will commence.

The internal power dissipation of the LM5035 can be re-

duced by powering VCC from an external supply. In typical

applications, an auxiliary transformer winding is connected

through a diode to the VCC pin. This winding must raise the

VCC voltage above 8.3V to shut off the internal start-up

regulator. Powering VCC from an auxiliary winding improves

efficiency while reducing the controller’s power dissipation.

The under-voltage comparator circuit will still function in this

mode, requiring that VCC never falls below 6.2V during the

start-up sequence.

During a fault mode, when the converter auxiliary winding is

inactive, external current draw on the VCC line should be

limited such that the power dissipated in the start-up regula-

tor does not exceed the maximum power dissipation of the

IC package.

An external DC bias voltage can be used instead of the

internal regulator by connecting the external bias voltage to

both the VCC and the VIN pins. The external bias must be

greater than 8.3V to exceed the VCC UVLO threshold and

less than the VCC maximum operating voltage rating (15V).

Line Under-Voltage Detector

The LM5035 contains a dual level Under-Voltage Lockout

(UVLO) circuit. When the UVLO pin voltage is below 0.4V,

the controller is in a low current shutdown mode. When the

UVLO pin voltage is greater than 0.4V but less than 1.25V,

the controller is in standby mode. In standby mode the VCC

and REF bias regulators are active while the controller out-

puts are disabled. When the VCC and REF outputs exceed

the VCC and REF under-voltage thresholds and the UVLO

pin voltage is greater than 1.25V, the outputs are enabled

and normal operation begins. An external set-point voltage

divider from VIN to GND can be used to set the minimum

operating voltage of the converter. The divider must be

designed such that the voltage at the UVLO pin will be

greater than 1.25V when VIN enters the desired operating

range. UVLO hysteresis is accomplished with an internal

23uA current sink that is switched on or off into the imped-

ance of the set-point divider. When the UVLO threshold is

exceeded, the current sink is deactivated to quickly raise the

voltage at the UVLO pin. When the UVLO pin voltage falls

below the 1.25V threshold, the current sink is enabled caus-

ing the voltage at the UVLO pin to quickly fall. The hysteresis

of the 0.4V shutdown comparator is internally fixed at

100mV.

The UVLO pin can also be used to implement various re-

mote 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 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 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 house-

keeping circuits. The regulator output is internally current

limited to 20mA (typical).

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