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APPLICATION INFORMATION

Typical Application

R2

C1

C2

C3

L1

R1

D1

2

3

5

4

AVIN

PVIN

FB

GND

SW

6

BOOST

UDG−04131

TPS40222

1

Voltage Setting

V

1 )

R1

R2

(3)

Output Filter (L1 and C3)

TPS40222

SLUS642A – OCTOBER 2005 – REVISED JANUARY 2006

Figure 9. Typical Application

The output voltage may be set by knowing that the feedback voltage is 0.8 V and using Equation 3. To determine

an output voltage, choose a convenient resistor value for R2 and calculate R1.

Since the loop compensation is internally fixed and cannot be changed, loop stability can only be controlled by

the proper choice of output inductance and capacitance. Table 2 provides a shortcut guide to this selection and

recommended capacitance to maintain a safe 50 degrees of phase margin for various inductors and output

voltages. The table also shows the minimum capacitance for 50 degress of phase margin at three temperatures,

with the worst case for stability at -40

°C. The granularity of the table is sufficient so the user can interpolate

between values to find a specific operating condition. The table values assume a full load, which is also the worst

case for phase compensation. As an example of using the table, consider a 2.5-V output converter with a 2.2-

µH

inductor. The table shows that a minimum of 15-

µF of output capacitance is required to guarantee greater than

50 degress of phase margin at the worst case temperature of -40

°C. With a lead capacitor (C

feedback as shown in Figure 10, this minimum capacitance increases to 26-

µF and the closed-loop frequency

increases by about 20%.

10