Drivers Off

UVLO

Disable

UVLO

7.6V/6.2 V

THERMAL

SHUTDOWN

Max

. Duty

Cycle Limiter

20 2A

VPWR

VCC

VCC

1.25V

R

1

R2

LM5032

LM5032

SNVS344B – MARCH 2005 – REVISED DECEMBER 2014

www.ti.com

7.3 Feature Description

7.3.1 Line Undervoltage Lock Out, UVLO, Shutdown

The LM5032 contains a line under-voltage lockout circuit (UVLO) designed to enable the VCC regulator and

output drivers when the system voltage (VPWR) exceeds the desired level (see Figure 14). VPWR is the voltage

normally applied to the transformer primary, and usually connected to the VIN pin (see the schematic on Page

1). The threshold at the UVLO comparator is 1.25V. An external resistor divider connected from VPWR to ground

provides 1.25V at the UVLO pin when VPWR is increased to the desired turn-on threshold. When VPWR is below

the threshold the VCC regulator and output drivers are disabled, and the internal 20 µA current source is off.

When VPWR reaches the threshold, the comparator output switches low to enable the internal circuits and the 20

µA current source. The 20 µA flows into the external divider’s junction, raising the voltage at UVLO, thereby

providing hysteresis. Internally the voltage at UVLO also drives the Maximum Duty Cycle Limiter circuit

(described below), which may influence the values chosen for the UVLO pin resistors. At maximum VPWR, the

voltage at UVLO should not exceed 6V. Refer to the Applications Information section for a procedure to calculate

the resistors values.

The LM5032 controller can be shutdown by forcing the UVLO pin below 1.25V with an external switch. When the

UVLO pin is low, the outputs and the VCC regulator are disabled, and the LM5032 enters a low power mode. If

VCC pin is not powered from an external source, the current into VIN drops to a nominal 500 µA. If the VCC pin

is powered from an external source, the current into VIN is nominally 50 µA, and the current into the VCC pin is

approximately 4.3 mA. To disable one regulator without affecting the other, see the description of the Soft-start

section.

Figure 14. Drivers Off and VCC Disable

7.3.2 Startup Regulator, VIN, VCC

The high voltage startup regulator is integral to the LM5032. The input pin VIN can be connected directly to a

voltage between 13V and 100V, with transient capability to 105V. The startup regulator provides bias voltages to

the series pass VCC regulator and the UVLO circuit. The VCC regulator is disabled until the voltage at the UVLO

pin (described above) exceeds 1.25V. For applications where VPWR exceeds 100V the internal startup regulator

can be powered from an external startup regulator or other available low voltage source. See the Applications

Information section for details.

The VCC under-voltage threshold circuit (UVT) monitors the VCC regulator output. When the series pass

regulator is enabled and the internal VCC voltage increases to > 7.6V, the UVT comparator activates the PWM

controller and output drivers via the Drivers Off signal. The UVT comparator has built-in hysteresis, with the lower

threshold nominally set to 6.2V. See Figure 1 and Figure 14.

When enabled, the VCC regulated output is 7.7V ±4% with current limited to a minimum of 19 mA (typically 22

mA). The regulator’s output impedance is

≊ 6Ω.

The VCC pin requires a capacitor to ground for stability, as well as to provide the surge currents to the external

MOSFETs via the gate driver outputs. The capacitor should be physically close to the VCC and GND pins.

In most applications it is necessary to power VCC from an external source as the average current required at the

output drivers may exceed the current capability of the internal regulator and/or the thermal capability of the

LM5032 package (see Figure 4). Normally the external source is derived from the converter’s power stage once

the LM5032 outputs are active. Refer to the Applications Information section for more information.

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