CM6800AGIP Champion Microelectronic Corp., CM6800AGIP Datasheet - Page 13

• Manufacturer Part Number: CM6800AGIP
• Description: LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO
• Manufacturer: Champion Microelectronic Corp.

where C
t
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
C
Caution should be exercised when using this minimum soft
start capacitance value because premature charging of the
SS capacitor and activation of the PWM section can result if
VFB is in the hysteresis band of the V
start-up. The magnitude of V
line voltage and nominal PFC output voltage. Typically, a
1.0
out to reach their nominal values prior to activation of the
PWM section at line voltages between 90Vrms and
265Vrms.
In today’s PC power supply, since it has the house-keeping
IC in the secondary, the time sequence maybe different
from the last paragraph. The 5mS delay and the 1uF Soft
Start Capacitor may vary and they may vary base on the
application conditions.
Generating V
After turning on CM6800A at 13V, the operating voltage
can vary from 10V to 17.9V. The threshold voltage of VCC
OVP comparator is 17.9V. The hysteresis of VCC OVP is
1.5V. When VCC see 17.9V, PFCOUT will be low, and
PWM section will not be disturbed. That’s the two ways to
generate VCC. One way is to use auxiliary power supply
around 15V, and the other way is to use bootstrap winding
to self-bias CM6800A system. The bootstrap winding can
be either taped from PFC boost choke or from the
transformer of the DC to DC stage.
2006/10/11
DEALY
SS
μ
= 5ms x
F soft start capacitor will allow time for V
is the desired start-up delay.
SS
is the required soft start capacitance, and the
Rev. 1.3
20
CC
0.7V
μ
A
= 142nF
FB
at start-up is related both to
SS
:
IN
OK comparator at
L
Champion Microelectronic Corporation
OW
FB
S
and PFC
TART-
U
P
C
The ratio of winding transformer for the bootstrap should be
set between 18V and 15V. A filter network is recommended
between VCC (pin 13) and bootstrap winding. The resistor of
the filter can be set as following.
R
I
If anything goes wrong, and VCC goes beyond 17.9V, the
PFC gate (pin 12) drive goes low and the PWM gate drive
(pin 11) remains function. The resistor’s value must be
chosen to meet the operating current requirement of the
CM6800A itself (5mA, max.) plus the current required by the
two gate driver outputs.
EXAMPLE:
With a wanting voltage called, V
and the CM6800A driving a total gate charge of 90nC at
100kHz (e.g. 1 IRF840 MOSFET and 2 IRF820 MOSFET),
the gate driver current required is:
I
R
R
Choose R
The CM6800A should be locally bypassed with a 1.0
ceramic capacitor. In most applications, an electrolytic
capacitor of between 47
across the part, both for filtering and as part of the start-up
bootstrap circuitry.
URRENT
OP
GATEDRIVE
FILTER
BIAS
BIAS
= 3mA (typ.)
=
=
x I
5mA
V
18V
VCC
= 100kHz x 90nC = 9mA
BIAS
BIAS
I
CC
PFC/PWM C
~ 2V, I
+
−
= 214Ω
+
−
15V
9mA
I
V
G
CC
VCC
= I
μ
OP
F and 220
+ (Q
BIAS
ONTROLLER
PFCFET
,of 18V, a VCC of 15V
μ
+ Q
CM6800A
F is also required
PWMFET
Page 13
C
) x fsw
OMBO
μ
F