Datasheet

www.ti.com
APPLICATION INFORMATION
Typical Application
R2
C1
C2
C3
L1
R1
D1
2 3
5 4
AVIN PVIN
FB GND SW
6
BOOST
V
OUT
V
IN
UDG−04131
TPS40222
1
Voltage Setting
V
OUT
 V
FB


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- µ H
inductor. The table shows that a minimum of 15- µ F 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
LEAD
) added to the
feedback as shown in Figure 10 , this minimum capacitance increases to 26- µ F and the closed-loop frequency
increases by about 20%.
10