Datasheet

VIN

GND

UVLO

LM5022

ON/OFF

UV2

UV1

2N7000 or

Equivalent

LM5022

SNVS480G –JANUARY 2007–REVISED DECEMBER 2013

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INPUT UNDER-VOLTAGE DETECTOR

The LM5022 contains an input Under Voltage Lock Out (UVLO) circuit. UVLO is programmed by connecting the

UVLO pin to the center point of an external voltage divider from VIN to GND. The resistor divider must be

designed such that the voltage at the UVLO pin is greater than 1.25V when VIN is in the desired operating

range. If the under voltage threshold is not met, all functions of the controller are disabled and the controller

remains in a low power standby state. UVLO hysteresis is accomplished with an internal 20 µA current source

that is switched on or off into the impedance of the set-point divider. When the UVLO threshold is exceeded, the

current source is activated to instantly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below

the 1.25V threshold the current source is turned off, causing the voltage at the UVLO pin to fall. The UVLO pin

can also be used to implement a remote enable / disable function. If an external transistor pulls the UVLO pin

below the 1.25V threshold, the converter will be disabled. This external shutdown method is shown in Figure 13.

Figure 13. Enable/Disable Using UVLO

ERROR AMPLIFIER

An internal high gain error amplifier is provided within the LM5022. The amplifier’s non-inverting input is internally

set to a fixed reference voltage of 1.25V. The inverting input is connected to the FB pin. In non-isolated

applications such as the boost converter the output voltage, V

, is connected to the FB pin through a resistor

divider. The control loop compensation components are connected between the COMP and FB pins. For most

isolated applications the error amplifier function is implemented on the secondary side of the converter and the

internal error amplifier is not used. The internal error amplifier is configured as an open drain output and can be

disabled by connecting the FB pin to ground. An internal 5 kΩ pull-up resistor between a 5V reference and

COMP can be used as the pull-up for an opto-coupler in isolated applications.

CURRENT SENSING AND CURRENT LIMITING

The LM5022 provides a cycle-by-cycle over current protection function. Current limit is accomplished by an

internal current sense comparator. If the voltage at the current sense comparator input exceeds 0.5V, the

MOSFET gate drive will be immediately terminated. A small RC filter, located near the controller, is

recommended to filter noise from the current sense signal. The CS input has an internal MOSFET which

discharges the CS pin capacitance at the conclusion of every cycle. The discharge device remains on an

additional 65 ns after the beginning of the new cycle to attenuate leading edge ringing on the current sense

signal.

Product Folder Links: LM5022