LM3492HC datasheet(13/29 Pages) TI1

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VIN

GND

REN1

REN2

VVIN

DEN

LM3492HC-Q1, LM3492HC

www.ti.com

SNVS797B – MARCH 2012 – REVISED OCTOBER 2015

Feature Description (continued)

8.3.2 LDO Regulator

The LM3492HC device offers an integrated, 5.5-V, LDO regulator. For stability, connect an external capacitor

CVCC of more than 0.47-µF between the VCC and GND pins. The current limit of the LDO is typically 30 mA. The

LDO regulator can be used to pullup the open-drain COMM pin with an external resistor, and sources current to

the ILIM pin to adjust the current limit of the integrated MOSFET. When the voltage on the VCC pin (VCC) is

higher than the undervoltage lockout (UVLO) threshold of 3.78 V, the device becomes enabled and the CDHC

pin sources a current to charge up an external capacitor (CCDHC) to provide a soft-start function.

8.3.3 Enable and Disable

To enable the LM3492HC device, the voltage on the EN pin (VEN) must be higher than an enable threshold of

typically 1.63 V. If the voltage on the EN pin (VEN) is lower than 1.43 V, the device shuts down. In this case, the

LDO regulator turns off and the CDHC pin becomes internally grounded. The EN pin internally pulls up. After

enable, a 40-µA current source pulls up the EN pin. If the EN pin is connected to low such that the device is

shutdown, the pullup current is reduced to 2 µA. These advantages allow the device to effectively avoid false

disabling by noise during operation, and minimize power consumption during shutdown. The enable threshold is

so precise that it can support a UVLO function for the input voltage as shown in Figure 22. The input voltage can

be connected to the EN pin through a resistor divider consisting of REN1 and REN2. This circuitry ensures that the

device operates after the input voltage reaches a minimum require value VIN(EN), as shown in Equation 1.

VIN(EN) = 1.63 V(1 + REN1/ REN2)

(1)

To maintain the VEN level below the absolute maximum specification, place a Zener diode (DEN) between the EN

pin and GND pins.

Figure 22. Input Voltage UVLO Implemented by Precision Enable

After the EN pin is pulled low, the device performs the following functions:

•

resets IOUT overvoltage and undervoltage indications and the corresponding COMM bit pattern

•

resumes the switching frequency tuning to the normal frequency

•

resumes channel 1 of the current regulator if it is disabled

Pulling the EN pin low for a short period of approximately 200 ns achieves these same functions with little or no

effect on the operation of the boost converter and the current regulator.

8.3.4 Current Limit

The current limit (ICL) of the integrated MOSFET of the LM3492HC device provides a cycle-by-cycle current limit

for protection. This limit can be decreased by injecting a small signal current, IILIM into the ILIM pin. The

relationship between ICL and IILIM is described in Equation 2.

ICL = ICL(max) – 4290 × IILIM

where

•

ICL(max) is the maximum current limit (3.9 A typical)

(2)

As shown in Figure 23, create current limit functionality by connecting a resistor (RILIM) between the VCC pin and

the ILIM pin. The typical voltage on the ILIM pin is 0.7 V. To obtain the maximum current limit, connect the ILIM

pin to ground.

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