UCC39422NG4 Datasheet, PDF (17/40 Pages) Texas Instruments – MULTIMODE HIGH-FREQUENCY PWM CONTROLLER

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UCC29421, UCC29422, UCC39421, UCC39422
MULTIMODE HIGHĆFREQUENCY PWM CONTROLLER
SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005
APPLICATION INFORMATION
relating peak inductor current to average output current for the flyback converter (continued)
ǒ Ǔ D +
VO
VIN ) VO
(11)
Substituting equations (10) and (11) into equation (9) yields:
ǒ ǒ ǓǓ ^{IPEAK +}
IOUT
1*
VO
VIN)VO
)₂
VIN
f
L
ǒ VO Ǔ
VIN ) VO
(12)
Figure 7 shows an example of a converter where both V_IN and V_OUT may be quite low in voltage. In this case,
a diode has been added to peak detect the voltage on the drain of the charge FET and use it for the pump input
voltage. This is used to drive the gates of the FETs. To assure that the pump voltage is used (rather than V_IN,
which may be low), resistor R_BIAS has also been added to the ISENSE input to inhibit LP mode. This technique
is discussed further in the section Changing the Low Power Threshold.
V_IN
1.8 TO 6.0V
+ C_IN
100µF
10V
10µF 16V
V_OUT
3.3V
10µF
+ ^10V
Q2
(P)
L1B
L1A
1k
Q1
Si9802
(N)
R_SENSE
0.05Ω
+V
IN
UCC39421
RSEN
1
RSEL 16
2 VOUT
0.1µF
3 VGRECT
4 PGND
COMP 15
FB 14
PFM 13
C_POLE
R_COMP ^CCOMP
5 CHRG
GND 12
RT
6 VPUMP
47k
RT 11
1M
0.1µF
7 CP
8 VIN
SYNC/SD 10
ISENSE 9
0.1µF
R3
100k
1%
R2
41k
1%
R1
20k
1%
SYNC
INPUT
R_SLOPE 1.5k
UDG−98214
Figure 8. Application Diagram for the SEPIC Technology Using a P-channel Synchronous Rectifier
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