tSS =

Vref x CSS

ISS

ICL

IPK

'I

IOCL

IOUT

Load Current

Increases

Normal Operation

Current Limited

12

LM3150

SNVS561G – SEPTEMBER 2008 – REVISED SEPTEMBER 2015

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Copyright © 2008–2015, Texas Instruments Incorporated

Feature Description (continued)

Figure 13. Inductor Current - Current Limit Operation

8.3.5 Short-Circuit Protection

The LM3150 controller will sense a short-circuit on the output by monitoring the output voltage. When the

feedback voltage has fallen below 60% of the reference voltage, Vref x 0.6 (≈ 0.36 V), short-circuit mode of

operation will start. During short-circuit operation, the SS pin is discharged and the output voltage will fall to 0 V.

The SS pin voltage, VSS, is then ramped back up at the rate determined by the SS capacitor and ISS until VSS

reaches 0.7 V. During this re-ramp phase, if the short-circuit fault is still present the output current will be equal

to the set current limit. Once the soft-start voltage reaches 0.7 V, the output voltage is sensed again and if the

VFB is still below Vref x 0.6 then the SS pin is discharged again and the cycle repeats until the short-circuit fault is

removed.

8.3.6 Soft-Start

The soft-start (SS) feature allows the regulator to gradually reach a steady-state operating point, which reduces

start-up stresses and current surges. At turnon, while VCC is below the undervoltage threshold, the SS pin is

internally grounded and VOUT is held at 0 V. The SS capacitor is used to slowly ramp VFB from 0 V to 0.6 V. By

changing the capacitor value, the duration of start-up can be changed accordingly. The start-up time can be

calculated using the following equation:

where

•

tSS is measured in seconds

•

Vref = 0.6 V

•

ISS is the soft-start pin source current, which is typically 7.7 µA (refer to Electrical Characteristics)

(11)

An internal switch grounds the SS pin if VCC is below the undervoltage lockout threshold, if a thermal shutdown

occurs, or if the EN pin is grounded. By using an externally controlled switch, the output voltage can be shut off

by grounding the SS pin.

During startup the LM3150 controller will operate in diode emulation mode, where the low-side gate LG will turn

off and remain off when the inductor current falls to zero. Diode emulation mode will allow start-up into a pre-

biased output voltage. When soft-start is greater than 0.7 V, the LM3150 controller will remain in continuous

conduction mode. During diode emulation mode at current limit the low-gate will remain off when the inductor

current is off.

The soft-start time should be greater than the input voltage rise time and also satisfy the following equality to

maintain a smooth transition of the output voltage to the programmed regulation voltage during start-up.

tSS ≥ (VOUT x COUT) / (IOCL - IOUT)

(12)