Texas Instruments TPS23751 Evaluation Module TPS23751EVM-104 TPS23751EVM-104 데이터 시트
제품 코드
TPS23751EVM-104
D
VC1
C
VC
ARTN
R
VC
V
C
T1
Bias WInding
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
250 kHz
17 nC
61.2mW
10
=
´
´
´
=
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.
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.
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
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
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.
Copyright © 2012–2014, Texas Instruments Incorporated
33
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