LM3489 datasheet(12/23 Pages) TI1

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VIN(UVLO) = VEN 1 +

LM3489

SNVS443A – MAY 2006 – REVISED JUNE 2008

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EXTERNAL SENSE RESISTOR

The VDS of a PFET will tend to vary significantly over temperature. This will result an equivalent variation in

current limit. To improve current limit accuracy an external sense resistor can be connected from VIN to the

source of the PFET, as shown in Figure 6. The current sense resistor, RCS should have value comparable with

RDSON of the PFET used, typically in the range of 50mΩ to 200 mΩ. The equation in the Current Limit Operation

section can be used by replacing the RDSON with RCS.

Figure 6. Current Sensing by External Resistor

PGATE

When switching, the PGATE pin swings from VIN (off) to some voltage below VIN (on). How far the PGATE will

swing depends on several factors including the capacitance, on time, and input voltage.

PGATE voltage swing will increase with decreasing gate capacitance. Although PGATE voltage will typically be

around VIN-5V, with very small gate capacitances, this value can increase to a typical maximum of VIN-8.3V.

Additionally, PGATE swing voltage will increase as on time increases. During long on times, such as when

operating at 100% duty cycle, the PGATE voltage will eventually fall to its maximum voltage of VIN-8.3V (typical)

regardless of the PFET gate capacitance.

The PGATE voltage will not fall below 0.4V (typical). Therefore, when the input voltage falls below approximately

9V, the PGATE swing voltage range will be reduced. At an input voltage of 7V, for instance, PGATE will swing

from 7V to a minimum of 0.4V.

DEVICE ENABLE, SHUTDOWN

The LM3489 can be remotely shutdown by forcing the enable pin to ground. With EN pin grounded, the internal

blocks other than the enable logic are de-activated and the shutdown current of the device will be lowered to only

7µA (typical). Releasing the EN pin allows for normal operation to resume. The EN pin is internally pulled high

with the voltage clamped at 8V typical. For normal operation this pin should be left open. In case an external

voltage source is applied to this pin for enable control, the applied voltage should not exceed the maximum

operating voltage level specified in this datasheet, i.e. 5.5V.

ADJUSTABLE UVLO

The under-voltage-lockout function can be implemented as shown in Figure 6. By incorporating the feature of the

internal enable threshold, the lockout level can be programmed through an external potential divider formed with

R3 and R4. The input voltage information is detected and compared with the enable threshold and the device

operation will be inhibited when VIN drops below the preset UVLO level. The UVLO and hysteresis voltage can

be calculated as follows:

(7)

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LM3489 Datasheet(PDF) 12 Page - Texas Instruments