Datasheet

D
VC1
C
VC
ARTN
R
VC
V
C
T1
Bias WInding
DISCHARGE RECHARGE
Hiccup Freque
1 1
F 37Hz
T T 4.9ms 22.3
ncy :
ms
= = =
+ +
DISCHARGE
DISCHARGE RECHARGE
T
4.9ms
Duty Cycle : D 18.0%
T T 4.9ms 22.3ms
= = =
+ +
( ) ( )
VC1 VC2 CUVH
DISCHARGE
TOTAL
C C V 10 F 0.47 F 3.2 V
T 4.9 ms
I 6.9 mA
+ ´ m + m ´
= = =
( ) ( )
VC1 VC2 CUVH
RECHARGE
VC _ ST
C C V 10 F 0.47 F 3.2 V
T 22.3ms
I 1.5mA
+ ´ m + m ´
= = =
( )
VC1 VC2 CUV
ST
VC _ ST
C C V
10.47 F 8.9 V
T 62.1ms
I 1.5mA
+ ´
m ´
= = =
SSD TOTAL
VC1 VC2
CUVH
T I
3.01ms 6.9mA
C C 6.49μF
V 3.2 V
´
´
+ = = =
GATE
DRIVE
C
TOTAL DRIVE VC _ OP
P
61.2mW
I 5.1mA
V 12 V
I I I 5.1mA 1.8mA 6.9mA
= = =
= + = + =
GATE
12
P 12 V 250kHz 17 nC 61.2mW
10
= ´ ´ ´ =
TPS23751
TPS23752
www.ti.com SLVSB97C –JULY 2012–REVISED JANUARY 2014
Compute gate drive power if V
C
is 12 V and Q
GATE
is 17 nC
(7)
This equation illustrates why MOSFET Q
G
should be an important consideration in selecting the switching
MOSFETs.
2. Estimate the required bias current at some intermediate voltage during the C
VC
discharge. For the TPS23751
and TPS23752, 12 V provides a reasonable estimate. Add the operating bias current to the gate drive
current.
(8)
3. Compute the required C
VC
based on startup within the typical softstart delay of 3.01 ms.
(9)
4. Choose a 10 μF electrolytic and 0.47 μF ceramic capacitor each rated for 16 V (minimum). Compute the
initial time to start the converter when operating from PoE. Using a typical bootstrap current of 1.5 mA,
compute the time to startup.
(10)
5. Compute the fault duty cycle and hiccup frequency
(11)
(12)
(13)
(14)
Switching Transformer Considerations and R
VC
Care in design of the transformer and V
C
bias circuit is required to obtain hiccup overload protection. Leading-
edge voltage overshoot on the bias winding may cause V
C
to peak-charge, preventing the expected tracking with
output voltage. Some method of controlling overshoot is usually required. The method may be as simple as a
series resistor, or an R-C filter in front of D
VC1
. Good transformer bias-to-output-winding coupling results in
reduced overshoot and better voltage tracking.
Figure 31. V
C
Pin Interface
R
VC
as shown in Figure 31 helps to reduce peak charging from the bias winding. Reduced peak charging
becomes especially important when tuning hiccup mode operation during output overload. Typical values for R
VC
are between 10 Ω and 100 Ω.
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