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TPS40055 Datasheet

  • TPS40055

  • 寬輸入 (8V-40V) 最高頻率 1MHz 的同步降壓控制器，具有輸出驅(qū)動(dòng)...

  • 634.57KB

  • 34頁

  • TI

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SLUS593B 鈭?DECEMBER 2003 鈭?REVISED APRIL 2004
TPS40054
TPS40055
TPS40057
APPLICATION INFORMATION
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 (5).
DV
+
DI
ESR
)
1
C
O
V
P*P
(5)
8
f
SW
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 (6).
E
L
+
1
2
where:
I
2
+
I
OH
2
L
I
2
(Joules)
(6)
*
I
OL
2
(Amperes)
2
(7)
D
I
OH
is the output current under heavy load conditions
D
I
OL
is the output current under light load conditions
Some applications may require an additional circuit to prevent false restarts at the UVLO voltage level. This
applies to applications which have high impedance on the input voltage line or which have excessive ringing
on the V
IN
line. The input voltage impedance can cause the input voltage to sag enough at start-up to cause
a UVLO shutdown and subsequent restart. Excessive ringing can also affect the voltage seen by the device and
cause a UVLO shutdown and restart. A simple external circuit provides a selectable amount of hysteresis to
prevent the nuisance UVLO shutdown.
Assuming a hysteresis current of 10% I
KFF
, and the peak detector charges to 8 V and V
IN(min)
= 10 V, the value
of R
A
is calculated by equation (8) using a R
KFF
= 71.5 k鈩?
R
A
+
R
KFF
0.1
(8
*
3.5)
V
IN(min)
*
3.5
+
495 kW
^
499 kW
(8)
C
A
is chosen to maintain the peak voltage between switching cycles. To keep the capacitor charge from
drooping 0.1-V, or from 8 V to 7.9 V.
C
A
+
(8
*
3.5)
R
A
7.9
f
SW
(9)
www.ti.com
13

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