Datasheet

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SLVS485 − AUGUST 2003
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15
Selecting the Output Capacitor
The value of the output capacitor depends on the output voltage ripple requirements as well as the maximum
voltage deviation during a load transient. The TPS6420x require a certain ESR value for proper operation. Low
ESR tantalum capacitors or PosCap work best in the application. A ceramic capacitor with up 1 µF may be used
in parallel for filtering short spikes. The output voltage ripple is a function of both the output capacitance and
the ESR value of the capacitor. For a switching frequency which is used with the TPS6420x, the voltage ripple
is typically between 90% and 95% due to the ESR value.
DV
pp
+ DI
ƪ
ESR )
ǒ
1
8 C
O
ƒ
Ǔ
ƫ
[ 1.1 DI ESR
(14)
ESR, max [
DV
pp
1.1 DI
(15)
The output capacitance typically increases with load transient requirements. For a load step from zero output
current to its maximum, the following equation can be used to calculate the output capacitance:
C
O
+
L DI
2
O
(V
I
* V
O
)xDV
(16)
Table 4. Capacitors Used in the Application
TYPE MANUFACTURER CAPACITANCE ESR VOLTAGE RATING
6TPB47M (PosCap) Sanyo 47 µF 0.1 Ω 6.3 V
T491D476M010AS Kemet 47 µF 0.8 Ω 10 V
B45197A Epcos 47 µF 0.175 Ω 16 V
B45294−R1107−M40 Epcos 100 µF 0.045 Ω 6.3 V
594D476X0016C2 Vishay 47 µF 0.11 Ω 16 V
Output Voltage Ripple
Output voltage ripple causes the output voltage to be higher or lower than set by the resistor divider at the
feedback pin. If the application runs with minimum on-time, the ripple (half of the peak-to-peak value) adds to
the output voltage. In an application which runs with minimum off-time, the output voltage is lower by the amount
of ripple (half of the peak-to-peak value) at the output.
Snubber Design
For low output current, the TPS6420x work in discontinuous current mode. When the current in the inductor
drops to zero, the inductor and parasitic capacitance form a resonant circuit, which causes oscillations when
both, diode and PMOS do not conduct at the end of each switching cycle. The oscillation can easily be damped
by a RC-snubber. The first step in the snubber design is to measure the oscillation frequency of the sine wave.
Then, a capacitor has to be connected in parallel to the Schottky diode which causes the frequency to drop to
half of its original value. The resistor is selected for optimum transient response (aperiodic).
R + 2pfL
f − measured resonant frequency
L − inductance used
(17)
Selecting the Right Device for the Application
The TPS6420x step-down controllers either operate with a fixed on-time or a fixed off-time control. It mainly
depends on the input voltage to output voltage ratio if the switching frequency is determined by the
minimum-on-time or the minimum-off-time. To select the right device for an application see the table below:
INPUT TO OUTPUT VOLTAGE RATIO
SWITCHING FREQUENCY
DETERMINED BY
PROPOSED DEVICE FOR
HIGH SWITCHING
FREQUENCY
PROPOSED DEVICE FOR
LOW SWITCHING
FREQUENCY
V
I
>> V
O
(e.g. V
I
= 5 V V
O
= 1.5 V) Minimum on−time TPS64203 TPS64200, TPS64201
V
I
≈ V
O
(e.g. V
I
= 3.8 V V
O
= 3.3 V) Minimum off−time TPS64202 TPS64200, TPS64201