Datasheet

VC CUV
ST
VC
C V
10.47 F 9 V
t = = = 23.6 ms
I 4 mA
´
m ´
STARTUP TOTAL
VC1 VC2
CUVH
t I
4 ms 5.6 mA
C + C = = = 6.4 F
V 3.5 V
´
´
m
DIS
DRIVE
DRIVE
C C
V
P
62.5 mW 7.5 V
I = x = x = 4.7 mA
V V 10 V 10 V
GAT2
10
P = 10 V 250 kHz 8 nC = 20 mW
10
´ ´ ´
GATE
10
P = 10 V 250 kHz 17 nC = 42.5 mW
10
´ ´ ´
C C
GATE C SW GATE GAT2 C SW GATE2
QG QG
V V
P = V f Q P = V f Q
V V
æ ö æ ö
´ ´ ´ ´ ´ ´
ç ÷ ç ÷
ç ÷ ç ÷
è ø è ø
DT
DT
t (ns)
100
R (k ) = = = 50
2 2
W
4 4
BLNK
SW
BIanking_Interval(%) 2
R (k ) = 10 = 10 = 80
f (kHz) 250
W ´ ´
TPS23757
www.ti.com SLVS948D –JULY 2009–REVISED NOVEMBER 2013
(a)
(b) Select R
BLNK
= 80.6 kΩ.
Dead Time Resistor, R
DT
The required dead time period depends on the specific topology and parasitics. To obtain the optimum timing
resistor, build the supply and tune the dead time to achieve the best efficiency after considering all corners of
operation (load, input voltage, and temperature). A good initial value is 100 ns. Program the dead time with a
resistor connected from DT to ARTN per Equation 3. Efficiency optimization may be performed by substituting a
potentiometer (POT) for R
DT
, and adjusting its value to obtain a minimum input current at the desired operating
load.
1. Choose R
DT
as follows assuming a t
DT
of 100 ns:
(a)
(b) Choose R
DT
= 49.9 kΩ
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/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)
(b) Compute gate drive power if V
C
is 10 V, the switching frequency is 250 kHz, Q
GATE
is 17 nC, and Q
GAT2
is 8 nC.
(c)
P
DRIVE
= 42.5 mW + 20 mW = 62.5 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
TPS23757, 7.5 V provides a reasonable estimate. Add the operating bias current to the gate drive current.
(a)
(b) I
TOTAL
= I
DRIVE
+ I
OPERATING
= 4.7 mA + 0.92 mA = 5.6 mA
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. In practice this is
conservative since it was assumed it would take the full 4 ms to start up.
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
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