LM46002PWPR datasheet(18/52 Pages) TI

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Enable

Internal SS Ramp

Ext Tracking Signal to SS pin

VOUT

SS/TRK

RTRT

RTRB

EXT RAMP

SSC

LM46002

SNVSA13B – APRIL 2014 – REVISED SEPTEMBER 2014

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Feature Description (continued)

8.3.6 Soft-Start and Voltage Tracking (SS/TRK)

The LM46002 has a flexible and easy to use start up rate control pin: SS/TRK. Soft-start feature is to prevent

inrush current impacting the LM46002 and its supply when power is first applied. Soft-start is achieved by slowly

ramping up the target regulation voltage when the device is first enabled or powered up.

The simplest way to use the device is to leave the SS/TRK pin open circuit or floating. The LM46002 will employ

the internal soft-start control ramp and start up to the regulated output voltage in 4.1 ms typically.

In applications with a large amount of output capacitors, or higher VOUT, or other special requirements, the soft-

start time can be extended by connecting an external capacitor CSS from SS/TRK pin to AGND. Extended soft-

start time further reduces the supply current needed to charge up output capacitors and supply any output

loading. An internal current source (ISSC = 2.2 µA) charges CSS and generates a ramp from 0 V to VFB to control

the ramp-up rate of the output voltage. For a desired soft start time tSS, the capacitance for CSS can be found by

(2)

The soft start capacitor CSS is discharged by an internal FET when VOUT is shutdown by hiccup protection or

ENABLE = logic low. When a large CSS is applied and ENABLE is toggled low only for a short period of time, it

could happen that CSS is not fully discharged and the next soft start ramp will follow internal soft start ramp

before reaching the left-over voltage on CSS and then follow the ramp programmed by CSS. If this is not

acceptable by a certain application, a R-C low-pass filter can be added to ENABLE to slow down the shutting

down of VCC, which allows more time to discharge CSS.

The LM46002 is capable of start up into prebiased output conditions. When the inductor current reaches zero,

the LS switch will be turned off to avoid negative current conduction. This operation mode is also called diode

emulation mode. It is built-in by the DCM operation in light loads. With a prebiased output voltage, the LM46002

will wait until the soft-start ramp allows regulation above the prebiased voltage and then follow the soft-start ramp

to the regulation level.

When an external voltage ramp is applied to the SS/TRK pin, the LM46002 FB voltage follows the ramp if the

ramp magnitude is lower than the internal soft-start ramp. A resistor divider pair can be used on the external

control ramp to the SS/TRK pin to program the tracking rate of the output voltage. The final voltage seen by the

SS/TRK pin should not fall below 1.2 V to avoid abnormal operation.

Figure 37. Soft Start Tracking External Ramp

VOUT tracked to an external voltage ramp has the option of ramping up slower or faster than the internal voltage

ramp. VFB always follows the lower potential of the internal voltage ramp and the voltage on the SS/TRK pin.

Figure 38 shows the case when VOUT ramps slower than the internal ramp, while Figure 39 shows when VOUT

ramps faster than the internal ramp. Faster start up time may result in inductor current tripping current protection

during start-up. Use with special care.

Figure 38. Tracking with Longer Start-up Time Than The Internal Ramp

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