LM25037, LM25037-Q1

SNVS572E – JULY 2008 – REVISED JANUARY 2016

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7.3 Feature Description

7.3.1 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 5.5 V to a maximum of 75 V. The regulator output at

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

greater than 0.45 V, 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 undervoltage (VCC UV) threshold of 5-V nominal, the internal voltage

reference (REF) reaches its regulation set point of 5 V and the UVLO voltage is greater than 1.25 V, 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 8.2 V 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-V 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

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

7.3.2 Line Undervoltage Detector

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

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

0.45 V but less than 1.25 V, 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 undervoltage thresholds and the UVLO pin voltage is greater than 1.25 V, 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 1.25 V 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 1.25-V threshold, the current source is activated to quickly raise the voltage at the

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

the voltage at the UVLO pin to quickly fall. The hysteresis of the 0.45-V 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.

7.3.3 Reference

The REF pin is the output of a 5-V 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).

7.3.4 Error Amplifier

An internal high gain error amplifier is provided within the LM25037. The amplifier’s noninverting reference is tied

to a 1.25-V reference. In non-isolated applications the power converter output is connected to the FB pin through

the voltage setting resistors and loop compensation is connected between the COMP and FB pins. A typical

gain/phase plot is shown in Typical Characteristics.

For most isolated applications the error amplifier function is implemented on the secondary side. Because the

internal error amplifier is configured as an open-drain output, it can be disabled by connecting FB to ground. The

internal 5-K pullup resistor connected between the COMP pin and the 5-V reference can be used as the pullup

for an opto-coupler or other isolation device.

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