TPS23754_17 Datasheet, PDF (31/45 Pages) Texas Instruments – High-Power/High-Efficiency PoE Interface and DC-DC Controller

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TPS23754, TPS23754-1, TPS23756
SLVS885H – OCTOBER 2008 – REVISED OCTOBER 2015
Typical Application (continued)
(a)
R_BLNK (k W) =
BIa n ki ng _ In te r va l( %)
f_SW (kHz)
´ 10⁴ = ²
250
´ 10⁴ = 80
(b) Select R_BLNK = 80.6 kΩ.
8.2.2.15 Estimating Bias Supply Requirements and C_VC
The bias supply (V_C) power requirements determine the C_VC sizing and frequency of hiccup during a fault. The
first step is to determine the power and current requirements of the power supply control, then use this to select
C_VC. The control current draw will be assumed constant with voltage to simplify the estimate, resulting in an
approximate value.
First determine the switching MOSFET gate drive power.
1. Let V_QG be the gate voltage swing that the MOSFET Q_G is rated to (often 10 V).
(a)
æ
P_GATE = V_C ´ f_SW ´ _ççè Q_GATE ´
V_C
V_QG
ö
÷÷ø
P_GAT2
= V_C
´ f_SW
æ
´ _ççè Q_GATE2
´
V_C
V_QG
ö
÷÷ø
(b) Compute gate drive power if V_C is 12 V, Q_GATE is 17 nC, and Q_GAT2 is 8 nC.
12
P_GATE = 12 V ´ 250 kHz ´17 nC ´ ₁₀ = 61.2 mW
12
(c) ^{PGAT2 = 12 V ´ 250 kHz ´ 8 nC ´ 10 = 28.8 mW}
P_DRIVE = 61.2 mW + 28.8 mW = 90 mW
(d) This 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 TPS23754
device, 12 V provides a reasonable estimate. Add the operating bias current to the gate drive current.
(a)
I_DRIVE
=
P_DRIVE
V_C
=
90 mW
12 V
= 7.5 mA
(b) I_TOTAL = I_DRIVE + I_OPERATING = 7.5 mA + 0.92 mA = 8.42 mA
3. Compute the required C_VC based on start-up within the typical soft-start period of 4 ms.
(a)
C_VC1 + C_VC2
=
T_STARTUP ´ I_TOTAL
V_CUVH
= ^{4 ms ´ 8.42 mA} = 5.18 mF
6.5 V
(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 start-up.
(b)
T_ST =
C_VC1 ´ V_CUV
I_VC
= ^{10.47 mF ´ 15 V}
4 mA
= 39 ms
5. Compute the fault duty cycle and hiccup frequency:
(a)
T_RECHARGE =
(C _VC1 + C_VC2 )
I_VC
´
V_CUVH
=
(10 mF + 0.47 mF) ´
4 mA
6.5 V
= 17 ms
(b)
T_DISCHARGE =
(C _VC1 + C_VC2 ) ´
I_TOTAL
V_CUVH
=
(10 mF + 0.47 mF)
8.42 m A
´
6.5V
= 8.08 ms
(a) The optocoupler current is 0 mA because the output is in current limit.
(b) Also, it is assumed I_T2P is 0 mA.
Duty Cycle: D =
T_DISCHARGE
=
8.08 ms
= 32%
(c)
T_DISCHARGE + T_RECHARGE 8.08 ms + 17 ms
Hiccup Frequency: F =
1
=
1
= 39.9 Hz
(d)
T_DISCHARGE + T_RECHARGE 8.08 ms + 17 ms
6. With the TPS23754 device, the voltage rating of C_VC1 and C_VC2 should be 25 V minimum while with the
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