Datasheet

APPLICATION INFORMATION
OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)
Fixed Output Voltage Version
ƒ
c
+
1
2p L C
O
Ǹ
+
1
2p 10 mH 10 m F
Ǹ
+ 15.9 kHz
with L = 10 µH, C
O
= 10 µF
Adjustable Output Voltage Version
V
out
+ 0.5 V
ǒ
1 )
R1
R2
Ǔ
C1 +
1
2 p ƒ
Z
R1
+
1
2 p 22 kHz R1
TPS62220 , , TPS62221 , , TPS62222
TPS62223 , TPS62224 , TPS62225
TPS62227 , TPS62228 , TPS62229
www.ti.com ....................................................................................................................................... SLVS491E – SEPTEMBER 2003 – REVISED FEBRUARY 2009
The TPS6222x series of step-down converter has internal loop compensation. Therefore, the external L-C filter
has to be selected to work with the internal compensation. This is especially important for the fixed output
voltage version. The adjustable output voltage version allows external capacitors across the feedback divider
resistors. This allows higher flexibility of the output filter selection when using the adjustable output voltage
device TPS62220.
The internal compensation is optimized to operate with an output filter of L = 10 µ H and C
O
= 10 µ F. Such an
output filter has its corner frequency at:
As a general rule of thumb, the product L × C should not move over a wide range when selecting a different output
filter. This is because the internal compensation is designed to work with a certain output filter corner frequency
as calculated above. This is especially important when selecting smaller inductor or capacitor values that move
the corner frequency to higher frequencies. However, when selecting the output filter a low limit for the inductor
value exists due to other internal circuit limitations. For the TPS6222x series the minimum inductor value should
be kept at 4.7 µ H. Selecting a larger output capacitor value is less critical because the corner frequency moves to
lower frequencies causing fewer stability problems. The possible output filter combinations are listed in Table 1 :
Table 1. Output Filter Combinations for Fixed Output Voltage Versions
V
O
L C
O
≤ 2 V 4.7 µ H ≥ 22 µ F (ceramic capacitor)
≤ 2 V 6.8 µ H ≥ 22 µ F (ceramic capacitor)
≤ 2 V 10 µ H ≥ 10 µ F (ceramic capacitor)
> 2 V 10 µ H 10 µ F (ceramic capacitor)
When the adjustable output voltage version TPS62220 is used, the output voltage is set by the external resistor
divider. See Figure 15 .
The output voltage is calculated as
with R1 + R2 ≤ 1 M Ω and internal reference voltage V
ref,typ
= 0.5 V
For stability, R1 + R2 should not be greater than 1 M Ω . To keep the operating quiescent current to a minimum,
the feedback resistor divider should have high impedance with R1 + R2 ≤ 1 M Ω . In general, for the adjustable
output voltage version, the same stability considerations are valid as for the fixed output voltage version.
Because the adjustable output voltage version uses an external feedback divider, it is possible to adjust the loop
gain using external capacitors across the feedback resistors. This allows a wider selection of possible output filter
components. This is shown in Figure 16. R1 and C1 places a zero in the loop and R2 and C2 places a pole in
the loop. The zero is calculated as:
with R1 = upper resistor of voltage divider, C1 = upper capacitor of voltage divider
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Product Folder Link(s): TPS62220 TPS62221 TPS62222 TPS62223 TPS62224 TPS62225 TPS62227 TPS62228
TPS62229