As a member of the SWIFT™ family of dc/dc regulators, the TPS54073 low-input voltage high-output current synchronous buck PWM converter integrates all required active components

TPS54073

Manufacturer Part NumberTPS54073
DescriptionAs a member of the SWIFT™ family of dc/dc regulators, the TPS54073 low-input voltage high-output current synchronous buck PWM converter integrates all required active components
ManufacturerTexas Instruments
TPS54073 datasheet
 


Specifications of TPS54073

Iout(max)(a)14Vin(min)(v)2.2
Vin(max)(v)4Vout(min)(v)0.9
Vout(max)(v)2.5Iq(typ)(ma)13
Switching Frequency(max)(khz)700Switch Current Limit(typ)(a)14.5
TopologyBuckOperating Temperature Range(c)-40 to 85
Pin/package28HTSSOPDuty Cycle(max)(%)90
Regulated Outputs(#)1  
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TPS54073
SLVS547 – FEBRUARY 2005
............................................................................................................................................................................................
V
V
x
OUT
I
I
=
+
L(PK)
OUT(MAX)
x
1.6
V
IN(MAX)
For this design, the RMS inductor current is 14.001 A,
and the peak inductor current is 14.43 A. The Vishay
IHLP5050CZ-01 style output inductor with a value of
2.2 H meets these current requirements. Increasing
the inductor value decreases the ripple current and
the corresponding output ripple voltage. The inductor
value can be decreased if more margin in the RMS
current is required. In general, inductor values for use
with the TPS54073 falls in the range of 1 H to 3.3
H, depending on the maximum required output
current.
Capacitor Requirements
The important design factors for the output capacitor
are dc voltage rating, ripple current rating, and
equivalent series resistance (ESR). The dc voltage
and ripple current ratings cannot be exceeded. The
ESR is important because along with the inductor
current it determines the amount of output ripple
voltage. The actual value of the output capacitor is
not critical, but some practical limits do exist.
Consider the relationship between the desired
closed-loop crossover frequency of the design and
LC corner frequency of the output filter. In general, it
is desirable to keep the closed-loop crossover
frequency at less than 1/5 of the switching frequency.
With high switching frequencies such as the 700 kHz
frequency of this design, internal circuit limitations of
the TPS54073 limit the practical maximum crossover
frequency to about 70 kHz. To allow for adequate
phase gain in the compensation network, the LC
corner frequency should be about one decade or so
below the closed-loop crossover frequency. This
limits the minimum capacitor value for the output filter
to:
1
K
C
+
OUT(MIN)
2p
L
OUT
CO
Where K is the frequency multiplier for the spread
between f
and f
. K should be between 5 and 15,
LC
CO
typically 10 for one decade difference. For a desired
crossover of 40-kHz and a 2.2- H inductor, the
minimum value for the output capacitor is 304
using a minimum K factor of 6.5. Increasing the K
factor would require using a larger capacitance. The
selected output capacitor must be rated for a voltage
greater than the desired output voltage plus one half
the ripple voltage. Any de-rating amount must also be
included. The maximum RMS ripple current in the
output capacitors is given by
Equation
12
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V
x
IN (MAX)
OUT
x
x
x
I
L
F sw
0.8
OUT
COUT(RMS)
(7)
The calculated RMS ripple current is 201 mA in the
output capacitors.
The maximum ESR of the output capacitor is
determined by the amount of allowable output ripple
as specified in the initial design parameters. The
output ripple voltage is the inductor ripple current
times the ESR of the output filter; therefore, the
maximum specified ESR as listed in the capacitor
data is given by
+ N
ESR
MAX
and the maximum ESR required is 29 mΩ. A
capacitor that meets these requirements is a Cornell
Sanyo POSCAP 6TPD33M rated at 6.3 V with a
maximum ESR of 0.015 Ω and a ripple current rating
of 2 A. An additional small 0.1- F ceramic bypass
capacitor C13 is a also used.
Other capacitor types work well with the TPS54073,
depending on the needs of the application.
Compensation Components
The external compensation used with the TPS54073
allows for a wide range of output filter configurations.
A large range of capacitor values and types of
dielectric are supported. The design example uses
Type-3 compensation consisting of R1, R3, R5, C6,
C7, and C8. Additionally, R2 along with R1 forms a
voltage divider network that sets the output voltage.
These component reference designators are the
same as those used in the SWIFT Designer
2
Software. There are a number of different ways to
design a compensation network. This procedure
outlines a relatively simple procedure that produces
(8)
good results with most output filter combinations. Use
the SWIFT Designer Software for designs with
unusually high closed-loop crossover frequencies,
low value, low ESR output capacitors such as
ceramics or if you are unsure about the design
F
procedure.
When designing compensation networks for the
TPS54073, a number of factors need to be
considered. The gain of the compensated error
amplifier should not be limited by the open-loop
amplifier gain characteristics and should not produce
excessive gain at the switching frequency. Also, the
9:
closed-loop crossover frequency should be set less
than one-fifth of the switching frequency, and the
Product Folder Link(s):
TPS54073
www.ti.com
* V
V
V
OUT
PVIN(MAX)
+ 1
V
L
12
PVIN(MAX)
OUT
Equation 10
:
V
L
F sw
0.8
IN(MAX)
OUT
DV
C
* V
V
V
OUT
IN(MAX)
OUT
Copyright © 2005, Texas Instruments Incorporated
OUT
F sw
(9)
P*P(MAX)
(10)