參數(shù)資料
型號(hào)： TPS53851RHHT
廠商： TEXAS INSTRUMENTS INC
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PQCC36
封裝： PLASTIC, VQFN-36
文件頁(yè)數(shù)： 39/66頁(yè)
文件大小： 1264K
代理商： TPS53851RHHT
÷
è
RIPPLE
RIPPLE(TotOUT)
RIPPLE(COUT)
OUT
SW
Co
RIPPLE
I
V
-
V
- V
8×C
× f
ESR
=
I
× V
OUT
C
=
IN(min)
V
× V
× f
RIPPLE(CIN)
IN
SW
VRIPPLE(CinESR)
ESR
=
Cin
1
I
+
I
OUT
RIPPLE
2
RMS_CIN
OUT
I
=
D × (1 - D) × I
I
SWrms +
D
I
OUT
2
)
I
RIPPLE
2
12
+ 7.07 A
(
)
(
)
(
)
( )
2
SW cond
SW rm s
DS on
P
I
R
0.295 W
=
=
SLUS985 – DECEMBER 2009
www.ti.com
(23)
With 880-
μF output capacitance, the ripple voltage at the capacitor is calculated to be 863-μV. In the
specification, the output ripple voltage should be less than 30 mV, so based on Equation 23, the required
maximum ESR is 9.5 m
. The selected capacitors can meet this requirement.
6.1.3
Step 3: Input Capacitor Selection
The input voltage ripple depends on the input capacitance and the ESR. The minimum capacitor and the
maximum ESR can be estimated by:
(24)
(25)
For this design, assume VRIPPLE(CIN) is 100 mV and VRIPPLE(CinESR) is 50 mV, so the calculated minimum
capacitance is 50
μF and the maximum ESR is 2.3 m. Choosing four 22-μF, 16-V, 2-m ESR ceramic
capacitors meets this requirement.
Another important consideration for the input capacitor is the RMS ripple current rating. The RMS current
in the input capacitor is estimated by:
(26)
D is the duty cycle. The calculated RMS current is 6.6 A. Each selected ceramic capacitor has a RMS
current rating of 4.3 A, so it is sufficient to reach this requirement.
6.1.4
Step 4: MOSFET Selection
The MOSFET selection determines the converter efficiency. In this design, the duty cycle is very small so
that the high-side MOSFET is dominated with switching losses and the low-side MOSFET is dominated
with conduction loss. To optimize the efficiency, choose smaller gate charge for the high-side MOSFET
and smaller RDS(on) for the low-side MOSFET.
The CSD16406Q3 and CSD16401Q5 are selected as the high-side and low-side MOSFETs, respectively.
The power losses in the high-side MOSFET is calculated with the following equations.
The RMS current in the high side MOSFET is show in Equation 27.
(27)
The RDS(on) is 5.9 m when the MOSFET gate voltage is 4.5 V.
The conduction loss is calculated in Equation 28.
(28)
The switching loss is:
44
DESIGN EXAMPLES
Copyright 2009, Texas Instruments Incorporated
Product Folder Link(s): TPS53851
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