ML4803
TYPICAL APPLICATIONS (Continued)
1
to develop the internal ramp by charging the internal
30pF +12/–10% capacitor. See Figures 10 and 11. The
frequency of the internal programming ramp is set
internally to 67kHz.
RCOMP
(3)
(4)
2
f
CCOMP
1
RCOMP
330k
6.28 30Hz 16nF
PFC CURRENT SENSE FILTERING
1
f
CZERO
In DCM, the input current wave shaping technique used
by the ML4803 could cause the input current to run away.
In order for this technique to be able to operate properly
under DCM, the programming ramp must meet the boost
inductor current down-slope at zero amps. Assuming the
programming ramp is zero under light load, the OFF-time
will be terminated once the inductor current reaches zero.
Subsequently the PFC gate drive is initiated, eliminating
the necessary dead time needed for the DCM mode. This
2
RCOMP
10
1
CZERO
0.16 F
6.28 3Hz 330k
INTERNALVOLTAGE RAMP
The internal ramp current source is programmed by way of
the VEAO pin voltage. Figure 7 displays the internal ramp
current vs. the VEAO voltage. This current source is used
forces the output to run away until the V OVP shuts
CC
down the PFC. This situation is corrected by adding an
60
Power Stage
Overall Gain
Compensation
Network Gain
40
20
V
O
0
11.3MΩ
VEAO
ML4803
I
ML4803
OUT
–20
–40
–60
R
667Ω
LOAD
220µF
330kΩ
15nF
I
VEAO
35µA
∆V
+
EAO
–
0.15µF
0.1
1
10
100
1000
POWER
STAGE
COMPENSATION
FREQUENCY (Hz)
Figure 4. Voltage Control Loop
Figure 5. Voltage Loop Gain
50
40
30
20
10
0
0
50
Power Stage
Overall
Compensation
Network
FF @ –55ºC
TYP @ –55ºC
TYP @ ROOM TEMP
TYP @ 155ºC
SS @ 155ºC
100
150
200
0
1
2
3
4
5
6
7
0.1
1
10
100
1000
V
(V)
FREQUENCY (Hz)
EAO
Figure 6. Voltage Loop Phase
Figure 7. Internal Ramp Current vs. V
EAO
8
February 1999
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
