Datasheet
( )
( )
( )
OUT1 1 1
RMS CIN1
I I D 1 D 1.5 A 0.413 1 0.413 0.74 A= ´ ´ - = ´ ´ - =
( )
( )
( )
OUT1 2 2
RMS CIN2
I I D 1 D 2.5 A 0.15 1 0.15 0.89 A= ´ ´ - = ´ ´ - =
FB FB
BIAS
OUT FB
V R
R
V V
´
=
-
( )
( )
f
6
ON
SW
TPS5429x
1.5 10 393ns
K t
6
IN OUT
6
600 kHz
FM 3762
V V
14 3.3
19.7 e 95 10
19.7 e 95 10
L
8.2 H
´ ´
´
-
-
= = =
é ù é ù
-
æ ö
æ ö
-
´ + ´ ´
´ + ´ ´
ê ú ê ú
ç ÷
ç ÷
m
è ø
ë û
è ø
ê ú
ë û
4
4
IN
C
6 6
IN
LOAD
V FM 2 10
14 V 3762 2 10
4.293
V FM 95 10 14 V 3762 95 10
1
1
4.4
2 R
-
-
- -
´ ´ ´
´ ´ ´
= = =
é ù é ù
æ ö æ ö
´ ´ ´ ´ ´ ´
ê ú ê ú
+
ç ÷ ç ÷
+
ç ÷
ç ÷
W
´
ê ú ê ú
è ø
è ø
ë û
ë û
f
( )
C
EA
CO LOAD OUT
3.22
K 20 log 20 log 11.83 dB
1 2 30kHz 4.4 22 F
1 2 f 2 R C
æ ö
æ ö
= - ´ = - ´ =
ç ÷
ç ÷
ç ÷
+ ´ p ´ ´ W ´ m
+ ´ p ´ ´ ´ ´
è ø
è ø
f
TPS54290, TPS54291, TPS54292
www.ti.com
SLUS973 –OCTOBER 2009
(32)
(33)
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 will have sufficient capacitance at the working voltage.
Feedback
The primary feedback divider resistor (R
FB
) from VOUT to FB should be selected between 10-kΩ and 100-kΩ to
maintain a balance between power dissipation and noise sensitivity. For a 3.3-V and 5-V output, 20.5 kΩ is
selected and the lower resistor is given by Equation 34.
(34)
For R
FB
= 20.5kΩ and V
FB
= 0.80 V, R
BIAS
= 6.56 kΩ and 41.0 kΩ (6.49 kΩ and 40.2 kΩ selected) for 3.3 V and
1.2 V respectively. It is common to select the next lower available resistor value for the bias resistor. This biases
the nominal output voltage slightly higher, allowing additional tolerance for load regulation.
Compensation Components
The TPS54291 controller uses a transconductance error amplifier, which is compensated with a series capacitor
and resistor to ground plus a high-frequency capacitor to reduce the gain at high frequency. To select the
component, the following equations define the control loop and power stage gain and transfer function:
(35)
where
• K = 5.6 × 10
5
for TPS54290
• K = 1.5 × 10
6
for TPS54291
• K = 3.6 × 10
6
for TPS54292
The overall DC gain of the converter control-to-output transfer function is approximated by Equation 36.
(36)
With the power stage DC gain, it is possible to estimate the required mid-band gain to program a desired
cross-over frequency.
(37)
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