RT =

x 6.25 x 10

9

1

¨

©

§

- 110 ns

In 1-

2.5V

VIN

¨

©

§

-1

2.5

Ps + 0.25 Ps

In 1-

2.5V

48V

¨

©

§

-1

=

= 51.4

Ps

LM5035

SNVS428H – JANUARY 2006 – REVISED OCTOBER 2015

www.ti.com

Feature Description (continued)

7.3.10 Feedforward Ramp and Volt-Second Clamp

the PWM ramp signal. The slope of the signal at RAMP will vary in proportion to the input line voltage. This

varying slope provides line feedforward information necessary to improve line transient response with voltage

mode control. The RAMP signal is compared to the error signal by the pulse width modulator comparator to

with the input voltage (VIN) to stabilize the volt-second product of the transformer primary signal. The power path

gain of conventional voltage-mode pulse-width modulators (oscillator generated ramp) varies directly with input

voltage. The use of a line generated ramp (input voltage feedforward) nearly eliminates this gain variation. As a

result, the feedback loop is only required to make very small corrections for large changes in input voltage.

In addition to the PWM comparator, a volt-second clamp comparator also monitors the RAMP pin. If the ramp

ramp capacitor is discharged by an internal 32-

Ω discharge MOSFET controlled by the volt-second clamp

comparator. If the RAMP signal does not exceed 2.5 V before the end of the clock period, the internal clock will

duration. The on-time set by the volt-second clamp varies inversely to the line voltage because the RAMP

(2.5 V). An example will illustrate the use of the volt-second clamp comparator to achieve a 50% duty cycle limit

at 200 kHz with a 48-V line input. A 50% duty cycle at a 200 kHz requires a 2.5-µs on-time. To achieve this

maximum on-time clamp level, use Equation 1:

(1)

with 110 k

Ω to approximate the desired 51.4-µs time constant. If load-transient response is slowed by the 10%

7.3.11 Oscillator, Sync Capability

The LM5035 oscillator frequency is set by a single external resistor connected between the RT and AGND pins.

To set a desired oscillator frequency, the necessary RT resistor is calculated from Equation 2:

(2)

For example, if the desired oscillator frequency is 400 kHz (HO and LO each switching at 200 kHz) a 15-k

Ω

resistor would be the nearest standard one percent value.

Each output (HO, LO, SR1 and SR2) switches at half the oscillator frequency. The voltage at the RT pin is

internally regulated to a nominal 2 V. The RT resistor must be located as close as possible to the IC and

connected directly to the pins (RT and AGND). The tolerance of the external resistor and the frequency tolerance

indicated in the Electrical Characteristics must be taken into account when determining the worst-case frequency

range.

16

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