LM5035C

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SNVS631C – JANUARY 2010 – REVISED MARCH 2013

FUNCTIONAL DESCRIPTION

The LM5035C PWM controller contains all of the features necessary to implement half-bridge voltage-mode

controlled power converters. The LM5035C provides two gate driver outputs to directly drive the primary side

power MOSFETs and two signal level outputs to control secondary synchronous 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 MOSFETs. Synchronous rectification allows higher conversion efficiency and greater

power density than conventional PN or Schottky rectifier techniques. The LM5035C 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

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 13.

High-Voltage Start-Up Regulator

The LM5035C contains an internal high voltage start-up regulator 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 58mA minimum. 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 driver (LO). When the voltage on

the VCC pin exceeds the UVLO threshold 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 LM5035C can be reduced 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 regulator 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 LM5035C 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 outputs 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 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. See the Soft-Start

section for more details.

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