Datasheet

Selecting C
FF
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Table 1. Suggested Components
V
OUT
R1 R2 C
FF
F
Z
0.80V 1.07 kΩ 1.78 kΩ 4700 pF 31.6 kHz
1.00V 1.00 kΩ 1.00 kΩ 4700 pF 33.8 kHz
1.20V 1.40 kΩ 1.00 kΩ 3300 pF 34.4 kHz
1.50V 2.00 kΩ 1.00 kΩ 2700 pF 29.5 kHz
1.80V 2.94 kΩ 1.13 kΩ 1500 pF 36.1 kHz
2.00V 1.02 kΩ 340Ω 4700 pF 33.2 kHz
2.50V 1.02 kΩ 255Ω 4700 pF 33.2 kHz
3.00V 1.00 kΩ 200Ω 4700 pF 33.8 kHz
3.30V 2.00 kΩ 357Ω 2700 pF 29.5 kHz
Refer to AN-1378 Method for Calculating Output Voltage Tolerances in Adjustable Regulators (SNVA112)
for additional information on how resistor tolerances affect the calculated V
OUT
value.
The LP38511MR-ADJ Evaluation board is assembled with a 1.40 kΩ ±1% resistor for R1, and a 1.00 kΩ
±1% resistor for R2. This sets V
OUT
to 1.20V.
V
OUT
= 500 mV x (1 + (1.40 kΩ / 1.00 kΩ ) ) (3)
5 Selecting C
FF
A capacitor placed across the gain resistor R1 will provide additional phase margin to improve load
transient response of the device. This capacitor, C
FF
, in parallel with R1, will form a zero in the loop
response given by the formula:
F
Z
= (1 / (2 x π x C
FF
x R1) ) (4)
The value for C
FF
should be selected to set a zero frequency (F
Z
) between 25 kHz and 50 kHz using the
formula:
C
FF
= 1 / (2 x π x F
Z
x R1) (5)
The closest standard 10% value is usually adequate for C
FF
.
The LP38511MR-ADJ Evaluation board is assembled with a 3300 pF capacitor for C
FF
. This sets F
Z
to
approximately 34 kHz.
F
Z
= (1 / (2 x π x 3300 pF x 1.40kΩ ) ) = 34.4 kHz (6)
Figure 1. 10 mA to 800 mA Load Transient Response
2
AN-1770 LP38511MR-ADJ Evaluation Board SNVA311B–March 2009–Revised April 2013
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