Datasheet

TPS62040
TPS62042, TPS62043
TPS62044, TPS62046
SLVS463B − JUNE 2003 − REVISED OCTOBER 2005
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13
APPLICATION INFORMATION
ADJUSTABLE OUTPUT VOLTAGE VERSION
When the adjustable output voltage version TPS62040 is used, the output voltage is set by the external resistor
divider. See Figure 15.
The output voltage is calculated as:
V
O
+ 0.5 V
ǒ
1 )
R1
R2
Ǔ
with R1 + R2 ≤ 1 MΩ and internal reference voltage V
ref
typical = 0.5 V
R1 + R2 should not be greater than 1 MΩ because of stability reasons. To keep the operating quiescent current to
a minimum, the feedback resistor divider should have high impedance with R1+R2≤1 MΩ. Due to this and the low
reference voltage of V
ref
= 0.5 V, the noise on the feedback pin (FB) needs to be minimized. Using a capacitive divider
C1 and C2 across the feedback resistors minimizes the noise at the feedback, without degrading the line or load
transient performance.
C1 and C2 should be selected as:
C1 +
1
2 p 10 kHz R1
with:
R1 = upper resistor of voltage divider
C1 = upper capacitor of voltage divider
For C1 a value should be chosen that comes closest to the calculated result.
C2 +
R1
R2
C1
with:
R2 = lower resistor of voltage divider
C2 = lower capacitor of voltage divider
For C2, the selected capacitor value should always be selected larger than the calculated result. For example, in
Figure 15 for C2 100 pF are selected for a calculated result of C2 = 88.42 pF.
If quiescent current is not a key design parameter C1 and C2 can be omitted, and a low impedance feedback divider
has to be used with R1 + R2 < 100 kΩ. This reduces the noise available on the feedback pin (FB) as well but increases
the overall quiescent current during operation. The higher the programmed output voltage the lower the feedback
impedance has to be for best operation when not using C1 and C2.
VIN
VIN
EN
MODE
SW
FB
PGND
PGND
SW
GND
2
3
1
49
10
5
7
8
6
TPS62040
C3
10 µF
V
I
2.5 V to 6 V
L1
10 µH
C4
10 µF
V
O
1.8 V / 1.2 A
R1
470 kΩ
R2
180 kΩ
C1
33 pF
C2
100 pF
Figure 15. Adjustable Output Voltage Version
(3)
(4)
(5)