Fp =

1

2 * S * (R1 R2) * (C1+C2)

Fz =

1

(2 * S * R1 * C1)

C2 =

1

(2 * S * R2 * 45 kHz)

C1 =

1

(2 * S * R1 * 45 kHz)

1 +

R1

R2

¹

·

©

§

LM3673

SNVS434L – JULY 2006 – REVISED OCTOBER 2015

www.ti.com

9.2.2 Detailed Design Procedure

9.2.2.1 Output Voltage Selection for LM3673-ADJ

The output voltage of the adjustable device can be programmed through the resistor network connected from

GND (R2) must be 200 k

Ω to keep the current drawn through this network well below the 16-µA quiescent

current level (PFM mode) but large enough that it is not susceptible to noise. If R2 is 200 k

the current through the resistor feedback network is 2.5 µA. The output voltage of the adjustable device ranges

from 1.1 V to 3.3 V.

The formula for output voltage selection is:

where

•

•

•

•

R2: feedback resistor from FB to GND

(2)

For any output voltage greater than or equal to 1.1 V, a zero must be added around 45 kHz for stability. The

formula for calculation of C1 is:

(3)

For output voltages higher than 2.5 V, a pole must be placed at 45 kHz as well. If the pole and zero are at the

same frequency the formula for calculation of C2 is:

(4)

The formula for location of zero and pole frequency created by adding C1 and C2 is shown in Equation 5 and

Equation 6. By adding C1, a zero as well as a higher frequency pole is introduced.

(5)

(6)

See Table 2.

14

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