Datasheet

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SLVS485 − AUGUST 2003
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14
Selecting the External PMOS
An external PMOS must be used for a step-down converter with the TPS64200. The selection criteria for the
PMOS are threshold voltage, r
DS(on)
, gate charge and current and voltage rating. Since the TPS64200 can
operate down to 1.8 V, the external PMOS must have a V
GS
(th) much lower than that if it is operated with such
a low voltage. As the gate of the PMOS finds the full supply voltage applied to the TPS64200, the PMOS must
be able to handle that voltage at the gate. The drain to source breakdown voltage rating should be at least a
few volts higher than the supply voltage in the application. The rms-current in the PMOS assuming low inductor
current ripple and continuous conduction mode, is:
I
PMOS(rms)
[ I
O
D
Ǹ
+ I
O
V
O
V
I
Ǹ
(11)
The power dissipated in the PMOS is comprised of conduction losses and switching losses. The conduction
losses are a function of the rms−current in the PMOS and the r
DS(on)
at a given temperature. They are calculated
using:
P
(cond)
+
ǒ
I
O
D
Ǹ
Ǔ
2
r
DS(on)
ǒ
1 ) TC
ƪ
T
J
–25°C
ƫ
Ǔ
[
ǒ
I
O
D
Ǹ
Ǔ
2
r
DS(on)
with TC = 0.005/°C
(12)
Table 2. PMOS Transistors Used in the Application Section
TYPE MANUFACTURER r
DS(on)
VDS ID PACKAGE
Si5447DC Vishay Siliconix 0.11 Ω at VGS = −2.5 V −20 V −3.5 A at 25°C 1206
Si5475DC Vishay Siliconix 0.041 Ω at VGS = −2.5 V −12 V −6.6 A at 25°C 1206
Si2301ADS Vishay Siliconix 0.19 Ω at VGS = −2.5 V −20 V −1.4 A at 25°C SOT23
Si2323DS Vishay Siliconix 0.41 Ω at VGS = −2.5 V −20 V −4.1 A at 25°C SOT23
FDG326P Fairchild 0.17 Ω at VGS = −2.5 V −20 V −1.5 A SC70
Selecting the Output Diode
The output diode conducts in the off phase of the PMOS and carries the full output current. The high switching
frequency demands a high-speed rectifier. Schottky diodes are recommended for best performance. Make sure
that the peak current rating of the diode exceeds the peak current limit set by the sense resistor R
(ISENSE)
or
r
DS(on)
. Select a Schottky diode with a low reverse leakage current to avoid an increased supply current. The
average current in the diode in continuous conduction mode, assuming low inductor current ripple, is:
I
(diode)(Avg)
[ I
O
(
1–D
)
+ I
O
ǒ
1–
V
O
V
I
Ǔ
(13)
Table 3. Tested Diodes
TYPE MANUFACTURER VR IF PACKAGE
MBRM120LT3 On Semiconductor 20 V 1 A DO216AA
MBR0530T1 On Semiconductor 30 V 0.5 A SOD123
ZHCS2000TA Zetex 40 V 2 A SOT23−6
B320 Diodes Inc. 20 V 3 A SMA