SLUU131A – September 2002 – Revised February 2003
6
TPS40001 Based Converter Delivers 10-A Output
4
Design Procedure
4.1
TPS4000X Family Device Selection
The TPS4000X family of devices offers four selections to encompass the frequency and
continuous/discontinuous inductor current options. The TPS4000/1 are selected for this high current application
because the 300-kHz switching frequency enables higher efficiency. The TPS40002/3 are available for
applications needing 600-kHz operation. The TPS4000X family also allows the user to select Discontinuous
Current Mode (DCM) operation or Continuous Current Mode (CCM) operation at lighter loads. In this reference
design the TPS40001 is selected to maintain continuous mode operation down to zero load. If desired, the
TPS40000 can be installed to turn the synchronous MOSFET off when the controller senses the inductor current
reaching zero, indicating the circuit is entering the DCM of operation.
4.2
Inductance Value
The output inductor value is selected to set the ripple current to a value most suited to overall circuit functionality.
An inductor selection that is too small leads to larger ripple current that increases RMS current losses in the
inductor and MOSFETs, and also leads to more ripple voltage on the output. The inductor value is calculated
by equation (1),
L
MIN
+
V
OUT
f
I
RIPPLE
ǒ
1
*
V
OUT
V
IN(max)
Ǔ
+
2.5 V
300 kHz
4 A
ǒ
1
*
2.5 V
5 V
Ǔ
+
1.0
m
H
in which I
RIPPLE
is chosen to be 40% of I
OUT
, or 4 A at max V
IN
. This high value of ripple current is selected to
keep the inductor small to minimize the R
DS(on)
losses due to the high output current. A synchronous rectifier
controller that maintains continuous inductor current down to no load eliminates concerns arising from crossing
the DCM boundary. A standard value of 1
µ
H with a resistance of 3.5 m
Ω
is selected. At full load the power loss
is only 0.35 W, which is only 1.4% of the 25-W output power.
4.3
Input Capacitor Selection
Bulk input capacitor selection is based on allowable input voltage ripple and required RMS current carrying
capability. In typical buck converter applications, the converter is fed from an upstream power converter with
its own output capacitance. In this standalone supply, onboard capacitance is added to handle input voltage
ripple and RMS current considerations. For this power level, input voltage ripple of 150 mV is reasonable, and
a conservative minimum value of capacitance is calculated as
C
+
I
D
t
D
V
+
10 A
2.5
m
s
0.15 V
+
167
m
F
In addition to this minimum capacitance requirement, the RMS current stresses must be considered. In this
converter, the large duty cycle causes the input RMS current to be nearly as large as the output current. This
simplified formula calculates the RMS current for a trapazoidal current waveform, shown in equation (3).
I
RMS
+
I
D
Ǹ +
I
V
OUT
V
IN(min)
Ǹ
+
10 A
2.5 V
3.0 V
Ǹ
+
9.1 A
Additional terms for the ripple component of the current add only a small amount to the total RMS current, and
can be neglected. To meet this initial requirement with small size and cost, a combination of capacitors is
considered. To carry the high frequency ripple current, three 22-
µ
F, X5R ceramic capacitors are placed close
to the power circuitry. Although these capacitors have an extremely small resistance, the datasheet indicates
that the part undergoes a 30
_
C temperature rise with 2 A
RMS
current at 500 kHz, so more current capability is
needed. Two 330-
µ
F POSCAPs with an RMS current capability of 4.4 A each is selected. In typical embedded
converters, these POSCAPs is not required if the upstream converter feeding this buck has sufficient current
handling capability.
(1)
(2)
(3)
