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參數資料
| 型號： | TPS40077PWPR |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 穩壓器 |
| 英文描述： | 2 A SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | GREEN, PLASTIC, HTSSOP-16 |
| 文件頁數： | 21/43頁 |
| 文件大小： | 847K |
| 代理商： | TPS40077PWPR |
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P
QSR + PCON ) PBD ) PGATE
(30)
P
CON + RDS(on)
1 *
V
OUT
V
IN
* t1 ) t2
f
S
Iout
2
)
I
RIPPLE
2
12
(31)
P
BD + Vf
I
OUT
t
1 ) t2
f
S
(32)
P
GATE + Qg(TOTAL)
Vg
f
S
(33)
R
T +
1
f
S
17.82
10*6
* 23
(34)
R
KFF + 0.131
R
T
V
UVLO(on) * 1.61
10*3
V
UVLO(on)
2
) 1.886
V
UVLO * 1.363 * 0.02
R
T
* 4.87
10*5
R
T
2
SLUS714D – JANUARY 2007 – REVISED APRIL 2009 ..................................................................................................................................................... www.ti.com
where
PBD = body diode losses
t1 = body diode conduction prior to turnon of channel = 12 ns for PGD
t2 = body diode conduction after turnoff of channel = 12 ns for PGD
Vf = body diode forward voltage
Estimating the body diode losses based on a forward voltage of 1 V gives 0.072 W. The gate losses are
unknown at this time, so assume 0.1-W gate losses. This leaves 0.428 W for conduction losses. Using this
figure, a target RDS(on) of 5 m was calculated.
The Si7336ADP from Vishay was chosen. Using the parameters from its data sheet, the actual expected power
losses are calculated. Conduction loss is 0.317 W, body diode loss is 0.072 W, and the gate loss is 0.136W. This
totals 0.525 W associated with the rectifier MOSFET.
Two other criteria should be verified before finalizing on the rectifier MOSFET. One is the requirement to ensure
that predictive gate drive functions correctly. The turnoff delay of the Si7336ADP is 97 ns. The minimum turnoff
delay of the Si7860DP is 25 ns. Together these devices meet the 130-ns requirement.
Secondly, the ratio between Cgs and Cgd should be greater than 1. The Si7336ADP easily meets this criterion.
This helps reduce the risk of dv/dt-induced turnon of the rectifier MOSFET. If this is likely to be a problem, a
small resistor may be added in series with the boost capacitor, CBOOST.
Component Selection for TPS40077
Timing Resistor, RT
The timing resistor is calculated using the following equation.
This gives a resistor value of 165 k
. The nominal frequency using this resistor is 300 kHz.
Feed-Forward and UVLO Resistor, RKFF
A resistor connected to the KFF pin of the IC feeds into the ramp generator. This resistor provides current into
the ramp generator proportional to the input voltage. The ramp is then adjusted to compensate for different input
voltages. This provides the voltage feed-forward feature of the TPS40077.
The same resistor also sets the undervoltage lockout point. The input start voltage should be used to calculate a
value for RKFF. For this module, the minimum input voltage is 8 V; however, due to tolerances in the IC, a start
voltage of 10% less than the minimum input voltage is selected. The start voltage for RKFF calculation is 7.2 V.
(35)
where RKFF and RT are in k.
This equation gives an RKFF value of 156 k. The closest lower standard value of 154 k should be selected.
This gives a minimum start voltage of 7.1 V.
28
Copyright 2007–2009, Texas Instruments Incorporated
Product Folder Link(s) :TPS40077
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