Datasheet

D
VC1
C
VC
ARTN
R
VC
V
C
T1
BiasWinding
DISCHARGE RECHARGE
1 1
Hiccup Frequency: F = = = 39.9 Hz
T + T 8.08 ms + 17 ms
DISCHARGE
DISCHARGE RECHARGE
T
8.08 ms
Duty Cycle: D = = = 32%
T + T 8.08 ms + 17 ms
VC1 VC2 CUVH
DISCHARGE
TOTAL
(C + C ) V
(10 F + 0.47 F) 6.5V
T = = = 8.08 ms
I 8.42 m A
´
m m ´
VC1 VC2 CUVH
RECHARGE
VC
(C + C ) V
(10 F + 0.47 F) 6.5 V
T = = = 17 m s
I 4 mA
´
m m ´
VC1 CUV
ST
VC
C V
10.47 F 15 V
T = = = 39 ms
I 4 m A
´
m ´
STARTUP TOTAL
VC1 VC2
CUVH
T I
4 ms 8.42 mA
C + C = = = 5.18 F
V 6.5 V
´
´
m
TPS23754
TPS23754-1
TPS23756
www.ti.com SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013
3. Compute the required C
VC
based on startup within the typical softstart period of 4 ms.
(a)
(b) For this case, a standard 10-μF electrolytic plus a 0.47 μF should be sufficient.
4. Compute the initial time to start the converter when operating from PoE.
(a) Using a typical bootstrap current of 4 mA, compute the time to startup.
(b)
5. Compute the fault duty cycle and hiccup frequency:
(a)
(b)
(a) Note that the optocoupler current is 0 mA because the output is in current limit.
(b) Also, it is assumed I
T2P
is 0 mA.
(c)
(d)
6. With the TPS23754 device, the voltage rating of C
VC1
and C
VC2
should be 25 V minimum while with the
TPS23756 rating can be 16 V.
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 this is usually required. This 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.
R
VC
as shown in Figure 32 helps to reduce peak charging from the bias winding. This becomes especially
important when tuning hiccup mode operation during output overload. Typical values for R
VC
will be between 10
Ω and 100 Ω.
Figure 32. R
VC
Usage
T2P Pin Interface
The T2P pin is an active low, open-drain output indicating a high-power source is available. An optocoupler is
typically used to interface with the T2P pin to signal equipment on the secondary side of the converter of T2P
status. Optocoupler current-gain is referred to as current transfer ratio (CTR), which is the ratio of transistor
collector current to LED current. To preserve efficiency, TI recommends a high-gain optocoupler ( 250% ≤ CTR ≤
500%, or 300% ≤ CTR ≤ 600% ) along with a high-impedance (for example, CMOS) receiver. Design of the T2P
optocoupler interface can be accomplished as follows:
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