hiccup

SS

1 V

T

(ms) C

(nF)

1 A

u

P

SS

SS

SS

V

1 V

T

(ms) C

(nF)

20 A

u

P

20

LM5025A

SNVS293F – DECEMBER 2004 – REVISED AUGUST 2016

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Copyright © 2004–2016, Texas Instruments Incorporated

Typical Application (continued)

RT = (5725/F)

1.026

where

•

F is in kHz and RT in kΩ

(6)

8.2.2.2 Soft-Start Ramp Time and Hiccup Interval

The soft-start ramp time and hiccup internal is programmed by a capacitor (CSS) on the SS pin to ground. The

soft-start ramp time is determined by comparing the SS pin voltage with COMP pin voltage. When the SS voltage

is less than COMP voltage, the COMP voltage is clamped by SS voltage. The PWM duty is limited by the

clamped COMP voltage, so that soft start can be achieved. The first PWM pulse is generated after COMP

voltage reaches 1 V. So the soft-start ramp time of the output voltage can be estimated by Equation 7:

where

•

VSS is the steady-state COMP pin voltage. This voltage is determined by the output voltage, voltage divider,

and the compensation network.

(7)

In hiccup mode, the SS current source is reduced to 1 µA. When the first PWM pulse is generated, the current

source switches to 20 µA, and the power supply tries to start up again. The hiccup interval can be calculated by

Equation 8:

(8)

8.2.2.3 Feedforward Ramp and Maximum On-Time Clamp

An example illustrates the use of the Volt × Second Clamp comparator to achieve a 50% duty cycle limit, at

200 KHz, at a 48-V line input: A 50% duty cycle at a 200 KHz requires a 2.5 µs of ON-time. At 48-V input the

Volt × Second product is 120 V × µs (48 V × 2.5 µs). To achieve this clamp level, see Equation 9 and

Equation 10:

RFF × CFF = VIN × TON / 2.5 V

(9)

48 × 2.5 µF / 2.5 = 48 µF

(10)

Select CFF = 470 pF

RFF = 102 kΩ

The recommended capacitor value range for CFF is 100 pF to 1000 pF.

8.2.2.4 Dead Times

The magnitude of the overlap and dead time can be calculated as follows in Equation 11 and Equation 12:

Overlap Time (ns) = 2.8 × RSET – 1.2

(11)

Dead Time (ns) = 2.9 × RSET + 20

where

•

RSET in kΩ, Time in ns

(12)