Datasheet

= ´ = ´ m =
Q IN Q
P V I 12 V 146 A 0.0018 W
GD IN G SW
P V Q 12 V 3nC 400 kHz 0.014 W= ´ ´ = ´ ´ =f
SW IN SW OUT rise
P V I t 12 V 400 kHz 5 A 4.9 ns 0.118 W= ´ ´ ´ = ´ ´ ´ =f
( )
( )
2
2
OUT
COND OUT
DS on
IN
V
5 V
P I R 5 A 92 m 0.958 W
V 12 V
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= ´ ´ = ´ W ´ =
ç ÷
è ø
1 1
C8 47.1 pF
R4 x sw x 16.9 k x 400 kHz x
= = =
p W pf
OUT ESR
C x R
87.4 F x 1.67 m
C8 8.64 pF
R4 16.9 k
m W
= = =
W
p(mod)
1 1
C5 5172 pF
2 R4 x 2 16.9 k x 1821 Hz
= = =
´ p ´ ´ p ´ Wf
co OUT OUT
REF
2 C V
2 29.2 kHz 87.4 5V
R4 x x 16.8 k
gmps V x gmea 17 A / V 0.8 V x 350 A / V
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´ p ´ ´
æ ö
æ ö
´ p ´ ´ m
æ ö
= = = W
ç ÷
ç ÷
ç ÷
ç ÷
m
è ø
è ø
è ø
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f
F
TPS54560
SLVSBN0A –MARCH 2013–REVISED MARCH 2014 www.ti.com
To determine the compensation resistor, R4, use Equation 46. Assume the power stage transconductance,
gmps, is 17 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 16.8 kΩ and a standard value of 16.9 kΩ is selected.
Use Equation 47 to set the compensation zero to the modulator pole frequency. Equation 47 yields 5172 pF for
compensating capacitor C5. 4700 pF is used for this design.
(46)
(47)
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 48 and Equation 49 for C8 to set the
compensation pole. The selected value of C8 is 47 pF for this design example.
(48)
(49)
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 408 mA. The power supply enters Eco-mode when the output current is lower than 25.3 mA. The
input current draw is 257 μA with no load.
9.2.2.11 Power Dissipation Estimate
The following formulas show how to estimate the TPS54560 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.
(50)
spacer
(51)
spacer
(52)
spacer
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).
• fsw is the switching frequency (Hz)
• trise is the SW terminal voltage rise time and can be estimated by trise = V
IN
x 0.16 ns/V + 3 ns
• Q
G
is the total gate charge of the internal MOSFET
• I
Q
is the operating nonswitching supply current (53)
Therefore,
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