ML4826

11

Volts peak. The composite waveform feeding the RAMP2

pin for the PWM consists of the reflected output current

signal along with the transformer magnetizing current and

the slope compensation signal.

Equation 8 describes the composite signal feeding

RAMP2, consisting of the primary current of the main

transformer and the slope compensation. Equation 9

solves for the required slope compensation peak voltage.

VI

V

L

N

N

T

n

VV

RAMP

PRI

OUT

S

P

S

CT

FB

2

1

2

1

15

=+ ×

×

×













×≤

–.

(8)

V

V

L

N

N

T

R

n

V

H

V

SC

OUT

S

P

S

SENSE

CT

=×

×

×













×= ×

×

×

=

1

2

1

2

48

20

14

90

5

471

200

22

µ

µsec

.

Ω

(9)

Soft Start

Start-up of the PWM is controlled by the selection of the

external capacitor at SS. A current source of 50

µA supplies

the charging current for the capacitor, and start-up of the

PWM begins at 1.5V. Start-up delay can be programmed

by the following equation:

Ct

A

V

SS

DELAY

=× 50

15

µ

.

(10)

It is important that the time constant of the PWM soft-start

allow the PFC time to generate sufficient output power for

the PWM section. The PWM start-up delay should be at

least 5ms.

Cms

A

V

nF

=

5

50

15

167

µ

.

(11)

The ML4826 is a current-fed part. It has an internal shunt

voltage regulator, which is designed to regulate the

voltage internal to the part at 13.5V. This allows a low

power dissipation while at the same time delivering 10V

of gate drive at the PWM OUT and PFC OUT outputs. It is

important to limit the current through the part to avoid

overheating or destroying the part.

ML2826. The method suggested in Figure 5, is one which

for a loosely regulated bootstrap winding. By feeding

external gate drive components from the base of Q1, the

constant current source does not have to account for

variations in the gate drive current. This helps to keep the

current available to charge the bootstrap capacitor from

the bootstrap winding is not limited by the constant

–

+

–

+

PWM CMP

1V

1.5V

R40

47.0k

Ω

R38

10.0k

Ω

R13

2.2k

Ω

RAMP2

AGND

U2

R21

8.63k

Ω

Q14

2N2222

D1

R16

471

Ω

C11

1000pF

C26

220pF

T3

200:1

x Former

4 x IN4148

6

10

11

9

7

18

17

Figure 4. Slope Compensation and Current Sense