Datasheet
C
O
+
L
ƪ
ǒ
I
OH
Ǔ
2
*
ǒ
I
OL
Ǔ
2
ƫ
ƪ
ǒ
V
f
Ǔ
2
*
ǒ
V
i
Ǔ
2
ƫ
(F)
V
2
+
ǒ
V
f
Ǔ
2
*
ǒ
V
i
Ǔ
2
ǒ
Volts
2
Ǔ
E
C
+
1
2
C V
2
(J)
I
2
+
ƪ
ǒ
I
OH
Ǔ
2
*
ǒ
I
OL
Ǔ
2
ƫ
ǒ
(
Amperes
)
2
Ǔ
E
L
+
1
2
L I
2
(J)
DV + DI
ƪ
ESR ) ǒ
1
8 C
O
f
SW
Ǔ
ƫ
ǒ
V
P*P
Ǔ
TPS40060
TPS40061
SLUS543F –DECEMBER 2002–REVISED JUNE 2013
www.ti.com
CALCULATING THE OUTPUT CAPACITANCE
The output capacitance depends on the output ripple voltage requirement, output ripple current, as well as any
output voltage deviation requirement during a load transient.
The output ripple voltage is a function of both the output capacitance and capacitor ESR. The worst case output
ripple is described in Equation 13.
(13)
The output ripple voltage is typically between 90% and 95% due to the ESR component.
The output capacitance requirement typically increases in the presence of a load transient requirement. During a
step load, the output capacitance must provide energy to the load (light to heavy load step) or absorb excess
inductor energy (heavy-to-light load step) while maintaining the output voltage within acceptable limits. The
amount of capacitance depends on the magnitude of the load step, the speed of the loop and the size of the
inductor.
Stepping the load from a heavy load to a light load results in an output overshoot. Excess energy stored in the
inductor must be absorbed by the output capacitance. The energy stored in the inductor is described in
Equation 14 and Equation 15.
(14)
where:
where:
• I
OH
is the output current under heavy load conditions
• I
OL
is the output current under light load conditions (15)
Energy in the capacitor is given by the following equation:
(16)
where:
where:
• V
f
is the final peak capacitor voltage
• V
i
is the initial capacitor voltage (17)
By substituting Equation 15 into Equation 14, substituting Equation 17 into Equation 16, setting Equation 14
equal to Equation 16 and solving for C
O
yields the following equation.
(18)
Loop Compensation
Voltage-mode buck-type converters are typically compensated using Type III networks. Since the TPS40060 and
TPS40061 use voltage feedforward control, the gain of the PWM modulator with voltage feedforward circuit must
be included. The generic modulator gain is described in Figure 8.
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