ML4826CP-1 Micro Linear, ML4826CP-1 Datasheet - Page 11

• Manufacturer Part Number: ML4826CP-1
• Description: PFC and Dual Output PWM Controller Combo
• Manufacturer: Micro Linear

FUNCTIONAL DESCRIPTION
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.
Soft Start
Start-up of the PWM is controlled by the selection of the
external capacitor at SS. A current source of 50 A 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:
where C
t
DELAY
V
V
C
SC
RAMP
SS
is the desired start-up delay.
SS
2
1
2
t
is the required soft start capacitance, and
V
DELAY
OUT
L
x Former
I
PRI
I
SENSE
N
N
S
P
1
2
T
50
S
1 5 .
V
OUT
L
T3
200:1
R
V
A
SENSE
n
CT
4 x IN4148
N
N
2
1
S
P
20
48
T
S
V
H
14
90
n
220pF
Figure 4. Slope Compensation and Current Sense
CT
1
C26
5
2N2222
sec
V
FB
Q14
471
D1
200
(Continued)
– .
1 5
2 2
R16
471
.
V
V
U2
(10)
R21
8.63k
(8)
(9)
C11
1000pF
2.2k
R13
47.0k
10.0k
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.
Solving for the minimum value of C
V
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.
There are a number of different ways to supply V
ML2826. The method suggested in Figure 5, is one which
keeps the ML4826 I
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
maximum I
current available to charge the bootstrap capacitor from
the bootstrap winding is not limited by the constant
CC
R40
R38
C
SS
17
18
11
10
7
9
6
5
V
V
R
RAMP2
AGND
DC I
V
CC
ms
T
CC
REF
DC
C
T
LIMIT
of the ML4826 to a minimum. Also, the
50
1 5
.
1V
CC
V
1.5V
A
current to a minimum, and allows
167
PWM CMP
DC I
–
+
–
+
nF
LIMIT
SS
:
ML4826
CC
to the
(11)
11