Datasheet

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Output Stage Component Selection
R1
31.25 kΩ
Internal
V
REF
800 mV
TPS40222
C0
2
3
5
4
AVIN
PVIN
FB
GND
SW
6
BOOST
+
L1
2.2 µH
R2
10 kΩ
C
LEAD
V
OUT
3.3 V
1.5 A
UDG−05095
1
V
OUT
 0.8 

1 
R1
R2

 3.3 V
(4)
Inductor Selection
L 
V  t
ON
I
(5)
TPS40222
SLUS642A – OCTOBER 2005 – REVISED JANUARY 2006
In most applications, the user starts with a known output voltage and current load requirement as shown in
Figure 10 .
Figure 10. Output Stage
As shown in Figure 10 , the trimmed reference voltage is internally connected to the error amplifier. Since the
input bias current to this error amplifier is negligible. The feedback resistors R1 and R2 can be selected over a
broad range limited by the low bias current into the error amplifier. With this restriction in mind, R2 was selected
at 10 k Ω , so its current is 80 µ A and large relative to the bias current of the error amplifier. The output voltage is
then given by Equation 4 .
where
• R1 = 31.25 k Ω
• R2 = 10 k Ω
This device's high-frequency internal clock enables the use of smaller, less expensive inductors. Ferrite, with its
good high frequency properties, is the material of choice. Several manufacturers provide catalogs with inductor
saturation currents, inductance values, and LSRs (internal resistance) for their various sized ferrites. For a 3.3-V,
1.5 A application, the inductor's saturation current must be higher than the maximum output current plus ½ the
ripple current. The inductor value sets the ripple current. A small inductor provides better transient response and
is a smaller, less expensive part. Too low an inductor value, however, causes high ripple currents that cause
high ripple voltage across the ESR of the output capacitance. A rule of thumb is to set high ripple current to be
less than 30% of the output current. A first order calculation then gives:
where
• ∆ V is the input voltage -( IR drops in the inductor and FET) - V
OUT
• ∆ I is 30% of 1.5 A
• t
ON
is the on time given by (V
OUT
/(V
IN
× f)) where f = 1.25 MHz
Under these conditions, L = 1.55 µ H
Selecting a standard value 2.2- µ H inductor, with an internal resistance of 32 m Ω , the peak current developed
during the on time is 1.66 A. This value is safely below the device's built in overcurrent limit of 2.1 A.
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