Datasheet

= ´ = ´ m =
Q IN Q
P V I 12 V 146 A 0.0018 W
f= ´ ´ = ´ ´ =
GD IN G SW
P V Q 12 V 3nC 600 kHz 0.022 W
f= ´ ´ ´ = ´ ´ ´ =
SW IN SW OUT rise
P V I t 12 V 600 kHz 3.5 A 4.9 ns 0.123 W
( )
( )
æ ö
= ´ ´ = ´ W ´ =
ç ÷
è ø
2
2
OUT
COND OUT
DS on
IN
V
3.3 V
P I R 3.5 A 92 m 0.31 W
V 12 V
f
= = =
p W p
1 1
C8 46.1 pF
R4 x sw x 11.5 k x 600 kHz x
m W
= = =
W
OUT ESR
C x R
70 F x 5 m
C8 30.4 pF
R4 11.5 k
f
= = =
´ p ´ ´ p ´ W
p(mod)
1 1
C5 5740 pF
2 R4 x 2 11.5 k x 2411 Hz
f
æ ö
´ p ´ ´
æ ö
æ ö
´ p ´ ´ m
æ ö
= = = W
ç ÷
ç ÷
ç ÷
ç ÷
m
è ø
è ø
è ø
è ø
F
co OUT OUT
REF
2 C V
2 26.9 kHz 70 3.3 V
R4 x x 11.6 k
gmps V x gmea 12 A / V 0.8 V x 350 A / V
TPS54340
SLVSBK0B –OCTOBER 2012–REVISED MARCH 2014 www.ti.com
To determine the compensation resistor, R4, use Equation 45. Assume the power stage transconductance,
gmps, is 12 A/V. The output voltage, V
O
, reference voltage, V
REF
, and amplifier transconductance, gmea, are 5
V, 0.8 V and 350 μA/V, respectively. R4 is calculated to be 11.6 kΩ and a standard value of 11.5 kΩ is selected.
Use Equation 46 to set the compensation zero to the modulator pole frequency. Equation 46 yields 5740 pF for
compensating capacitor C5. 5600 pF is used for this design.
(45)
(46)
A compensation pole can be implemented if desired by adding capacitor C8 in parallel with the series
combination of R4 and C5. Use the larger value calculated from Equation 47 and Equation 48 for C8 to set the
compensation pole. The selected value of C8 is 47 pF for this design example.
(47)
(48)
9.2.2.10 Discontinuous Conduction Mode and Eco-mode Boundary
With an input voltage of 12 V, the power supply enters discontinuous conduction mode when the output current
is less than 342 mA. The power supply enters Eco-mode when the output current is lower than 31.4 mA. The
input current draw is 237 μA with no load.
9.2.2.11 Power Dissipation
The following formulas show how to estimate the TPS54340 power dissipation under continuous conduction
mode (CCM) operation. These equations should not be used if the device is operating in discontinuous
conduction mode (DCM).
The power dissipation of the IC includes conduction loss (P
COND
), switching loss (P
SW
), gate drive loss (P
GD
) and
supply current (P
Q
). Example calculations are shown with the 12 V typical input voltage of the design example.
(49)
spacer
(50)
spacer
(51)
spacer
(52)
Where:
I
OUT
is the output current (A).
R
DS(on)
is the on-resistance of the high-side MOSFET (Ω).
V
OUT
is the output voltage (V).
V
IN
is the input voltage (V).
ƒsw is the switching frequency (Hz).
trise is the SW terminal voltage rise time and can be estimated by trise = V
IN
x 0.16ns/V + 3.0ns.
Q
G
is the total gate charge of the internal MOSFET.
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