Datasheet

DRIVE
DRIVE
C
P
90 mW
I = = = 7.5 mA
V 12 V
G AT2
12
P = 12 V 250 kHz 8 nC = 28.8 mW
10
´ ´ ´
GATE
12
P = 12 V 250 kHz 17 nC = 61.2 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 ´ ´
TPS23754
TPS23754-1
TPS23756
SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013 www.ti.com
Blanking Period, R
BLNK
Selection of the blanking period is often empirical because it is affected by parasitics and thermal effects of every
device between the gate-driver and output capacitors. The minimum blanking period prevents the current limit
and PWM comparators from being falsely triggered by the inherent current spike that occurs when the switching
MOSFET turns on. The maximum blanking period is bounded by the output rectifier's ability to withstand the
currents experienced during a converter output short.
If blanking beyond the internal default is desired choose R
BLNK
using R
BLNK
(kΩ) = t
BLNK
(ns).
1. For a 100 ns blanking interval:
(a) R
BLNK
(kΩ) = 100
(b) Choose R
BLNK
= 100 kΩ.
The blanking interval can also be chosen as a percentage of the switching period.
1. Compute R
BLNK
as follows for 2% blanking interval in a switcher running at 250 kHz.
(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. The easiest technique to obtain
the optimum timing resistor is to 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.
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 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)
(b) Compute gate drive power if V
C
is 12 V, Q
GATE
is 17 nC, and Q
GAT2
is 8 nC.
(c)
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)
(b) I
TOTAL
= I
DRIVE
+ I
OPERATING
= 7.5 mA + 0.92 mA = 8.42 mA
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