LM5035AMHX datasheet(13/30 Pages) NSC

13 / 30 page

Functional Description

The LM5035A PWM controller contains all of the features

necessary to implement half-bridge voltage-mode controlled

power converters. The LM5035A provides two gate driver

outputs to directly drive the primary side power MOSFETs

and two signal level outputs to control secondary syn-

chronous 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 ef-

ficiency and greater power density than conventional PN or

Schottky rectifier techniques. The LM5035A can be config-

ured 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 capability,

precision reference, thermal shutdown and programmable

volt•second clamping. These features simplify the design of

voltage-mode half-bridge DC-DC power converters. The

Functional Block Diagram is shown in Figure 1.

High-Voltage Start-up Regulator

The LM5035A contains an internal high voltage start-up reg-

ulator 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 35 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 values

for the VCC capacitor is 0.1 µF to 100 µF.

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 LM5035A 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 reg-

ulator. 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 lim-

ited such that the power dissipated in the start-up regulator

does not exceed the maximum power dissipation of the IC

package.

An external DC bias voltage can be used instead of the inter-

nal 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 LM5035A 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 con-

troller is in standby mode. In standby mode the VCC and REF

bias regulators are active while the controller outputs are dis-

abled. 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 op-

eration 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 ac-

complished with an internal 23 µA current sink that is switched

on or off into the impedance 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 causing the voltage at the UVLO pin to quickly fall.

The hysteresis of the 0.4V shutdown comparator is internally

fixed at 100 mV.

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 LM5035A 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 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 house-

keeping circuits. The regulator output is internally current

limited to 20mA (typical).

LM5035AMHX Datasheet(PDF) 13 Page - National Semiconductor (TI)

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