SLUS593B 鈭?DECEMBER 2003 鈭?REVISED APRIL 2004
TPS40054
TPS40055
TPS40057
APPLICATION INFORMATION
The value of C
A
imay calculate to less than 10 pF, but some standard value up to 470 pF works adequately. The
diode can be a small signal switching diode or Schottky rated for more then 20 V. Figure 5 illustrates a typical
implementation using a small switching diode.
The tolerance on the UVLO set point also affects the maximum duty cycle achievable. If the UVLO starts the
device at 10% below the nominal start up voltage, the maximum duty cycle is reduced approximately 10% at
the nominal start up voltage.
+
VIN
鈭?/div>
RA
499 kW
RKFF
71.5 kW
TPS40050PWP
TPS40051PWP
1 KFF
CA
470 pF
2 RT
3 BP5
4 SYNC
5 SGND
6 SS
7 VFB
8 COMP
ILIM 16
VIN 15
BOOST 14
HDRV 13
SW 12
BP10 11
LDRV 10
PGND 9
DA
1N914, 1N4150
Type Signal Diode
UDG鈭?3034
PWP
Figure 8. Hysteresis for Programmable UVLO
Energy in the capacitor is described in equation (10).
E
C
+
1
2
where:
V
2
+
where:
V
f
2
C
V
2
(Joules)
(10)
*
V
i
2
Volts
2
(11)
D
V
f
is the final peak capacitor voltage
D
V
i
is the initial capacitor voltage
Substituting equation (7) into equation (6), then substituting equation (11) into equation (10), then setting
equation (10) equal to equation (6), and then solving for C
O
yields the capacitance described in equation (12).
L
C
O
+
V
f
I
OH
2
2
*
I
OL
2
2
(Farads)
*
V
i
(12)
14
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