LT1961
U
W U U
APPLICATIONS INFORMATION
FB RESISTOR NETWORK
defines the pole frequency of the output stage, an X7R or
X5R type ceramic, which have good temperature stability,
is recommended.
The suggested resistance (R2) from FB to ground is 10k
1%. This reduces the contribution of FB input bias current
to output voltage to less than 0.2%. The formula for the
resistor (R1) from VOUT to FB is:
Tantalum capacitors are usually chosen for their bulk
capacitance properties, useful in high transient load appli-
cations. ESR rather than absolute value defines output
ripple at 1.25MHz. Values in the 22μF to 100μF range are
generally needed to minimize ESR and meet ripple current
ratings. Care should be taken to ensure the ripple ratings
are not exceeded.
R2 VOUT −1.2
(
)
R1=
1.2 − R2(0.2μA)
Table 1. Surface Mount Solid Tantalum Capacitor ESR and
Ripple Current
V
LT1961
SW
OUTPUT
E Case Size
ESR (Max,
Ω
)
Ripple Current (A)
0.7 to 1.1
0.4
ERROR
AMPLIFIER
AVX TPS, Sprague 593D
AVX TAJ
0.1 to 0.3
0.7 to 0.9
1.2V
+
–
R1
+
FB
D Case Size
AVX TPS, Sprague 593D
C Case Size
0.1 to 0.3
0.2 (typ)
0.7 to 1.1
0.5 (typ)
R2
10k
1961 F02
AVX TPS
V
C
GND
Figure 2. Feedback Network
INPUT CAPACITOR
Unlike the output capacitor, RMS ripple current in the
input capacitor is normally low enough that ripple current
rating is not an issue. The current waveform is triangular,
with an RMS value given by:
OUTPUT CAPACITOR
Step-up regulators supply current to the output in pulses.
The rise and fall times of these pulses are very fast. The
output capacitor is required to reduce the voltage ripple
this causes. The RMS ripple current can be calculated
from:
0.29 V VOUT − V
(
IN)(
IN
)
IRIPPLE RMS
=
(
)
L f VOUT
( )( )(
)
IRIPPLE RMS = IOUT VOUT − V
/
V
IN
(
IN
)
(
)
Athigherswitchingfrequency,theenergystoragerequire-
ment of the input capacitor is reduced so values in the
range of 1μF to 4.7μF are suitable for most applications.
Y5V or similar type ceramics can be used since the
absolute value of capacitance is less important and has no
significant effect on loop stability. If operation is required
close to the minimum input voltage required by either the
output or the LT1961, a larger value may be necessary.
This is to prevent excessive ripple causing dips below the
minimum operating voltage resulting in erratic operation.
The LT1961 will operate with both ceramic and tantalum
output capacitors. Ceramic capacitors are generally cho-
sen for their small size, very low ESR (effective series
resistance), and good high frequency operation, reducing
outputripplevoltage. TheirlowESRremovesausefulzero
in the loop frequency response, common to tantalum
capacitors. To compensate for this, the VC loop compen-
sationpolefrequencymusttypicallybereducedbyafactor
of 10. Typical ceramic output capacitors are in the 1μF to
10μF range. Since the absolute value of capacitance
1961fa
7
Linear [ Linear ]
