LM25037, LM25037-Q1

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SNVS572C – JULY 2008 – REVISED JANUARY 2010

FUNCTIONAL DESCRIPTION

The LM25037 PWM controller contains all the features necessary to implement double-ended power converter

topologies such as push-pull, half-bridge and full-bridge. The unique architecture allows the modulator to be

configured for either voltage-mode or current-mode control. The LM25037 provides two alternative gate driver

outputs to drive the primary side power MOSFETs with programmable forced dead-time. The LM25037 can be

configured to operate with bias voltages ranging from 5.5V to 75V. Additional features include line under-voltage

lockout, cycle-by-cycle current limit, voltage feed-forward compensation, and hiccup mode fault protection with

adjustable delays, soft-start, and a 2MHz capable oscillator with synchronization capability, precision reference

and thermal shutdown. These rich set of features simplify the design of double ended topologies. The functional

block diagram is shown in Figure 14.

HIGH-VOLTAGE START-UP REGULATOR

The LM25037 contains an internal high voltage, low drop-out start-up regulator that allows the input pin (VIN) to

be connected directly to the supply voltage over a range of 5.5V to a maximum of 75V. The regulator output at

VCC (7.7V) is internally current limited with a guaranteed minimum of 20 mA. When the UVLO pin potential is

greater than 0.45V, 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 gate drivers (OUTA and OUTB). When the

voltage on the VCC pin exceeds its Under Voltage (VCC UV) threshold of 5.0V nominal, the internal voltage

reference (REF) reaches its regulation set point of 5V and the UVLO voltage is greater than 1.25V, the controller

outputs are enabled. The value selected for the VCC capacitor depends on the total system design, and its start-

up characteristics. The recommended range of values for the VCC capacitor is 0.47 µF to 10 µF.The internal

power dissipation of the LM25037 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 8.2V to shut off the internal start-up regulator. Powering VCC from an auxiliary

winding improves efficiency while reducing the controller’s power dissipation. The VCC UV circuit will still function

in this mode, requiring that VCC never falls below 5.0V nominal during the start-up sequence. The VCC regulator

series pass transistor includes a diode between VCC and VIN that should not be forward biased in normal

operation. Therefore the auxiliary VCC voltage should never exceed the VIN voltage.

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. In this particular case, the external bias must be greater than max VCC UV of

5.4V and less than the VCC maximum operating voltage rating (14V).

LINE UNDER-VOLTAGE DETECTOR

The LM25037 contains a dual level line Under-Voltage Lock Out (UVLO) circuit. When the UVLO pin voltage is

below 0.45V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.45V

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 their respective 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 22 µA current source that is switched on or off into the impedance of the set-point divider. When the

UVLO pin voltage exceeds 1.25V threshold, the current source is activated to quickly raise the voltage at the

UVLO pin. When the UVLO pin voltage falls below the 1.25V threshold, the current source is disabled causing

the voltage at the UVLO pin to quickly fall. The hysteresis of the 0.45V shutdown comparator is internally fixed at

100 mV.

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

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

more details.

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 10 mA (typical).

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Product Folder Links: LM25037 LM25037-Q1