TPS54383, TPS54386
www.ti.com
SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
2
)
æ
ç
ö
÷
æ
ç
ç
è
ö
÷
÷
ø
DI
(
2
)
OUTPUTx
I
=
D ´
I
(
+
RMS(outputx)
OUTPUTx
ç
ç
è
÷
÷
ø
12
(38)
•
IRMS(CIN) = 0.43 A
One 1210 10-µF, 25 V, X5R ceramic capacitor with 2-mΩ ESR and a 2-A RMS current rating are selected for
each PVDD input. Higher voltage capacitors are selected to minimize capacitance loss at the DC bias voltage to
ensure the capacitors maintain sufficient capacitance at the working voltage.
Boot Strap Capacitor
To ensure proper charging of the high-side FET gate and limit the ripple voltage on the boost capacitor, a 33-nF
boot strap capacitor is used.
ILIM
Current limit must be set above the peak inductor current IL(peak). Comparing IL(peak) to the available minimum
current limits, ILIM is connected to BP for the highest current limit level.
SEQ
The SEQ pin is left floating, leaving the enable pins to function independently. If the enable pins are tied
together, the two supplies start-up ratiometrically. Alternatively, SEQ could be connected to BP or GND to
provide sequential start-up.
Power Dissipation
The power dissipation in the TPS54383 is composed of FET conduction losses, switching losses and internal
regulator losses. The RMS FET current is found using Equation 39.
2
)
æ
ç
ö
÷
æ
ç
ç
è
ö
÷
÷
ø
DI
(
2
)
OUTPUTx
I
=
D ´
I
(
+
RMS(outputx)
OUTPUTx
ç
ç
è
÷
÷
ø
12
(39)
(40)
This results in 1.05-A RMS for Channel 1 and 0.87-A RMS for Channel 2.
Conduction losses are estimated by:
2
P
= R
´ I
CON
(
DS on QSW rms
(
)
( )
)
Conduction losses of 198 mW and 136 mW are estimated for Channel 1 and Channel 2 respectively.
The switching losses are estimated in Equation 41.
2
V
(
»
´ C + C
´ f
)
OSS SW
(
)
DJ
IN max
(
)
P
SW
2
(41)
From the data sheet of the MBRS320, the junction capacitance is 658 pF. Since this is large compared to the
output capacitance of the TPS54x8x the FET capacitance is neglected, leaving switching losses of 17 mW for
each channel.
The regulator losses are estimated in Equation 42.
PREG » IDD ´ V
+ IBP ´ V
- VBP
IN max
)
)
(
IN max
(
)
(
(42)
With no external load on BP (IBP=0) the regulator power dissipation is 66 mW.
Total power dissipation in the device is the sum of conduction and switching for both channels plus regulator
losses.
Copyright © 2007, Texas Instruments Incorporated
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37
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