Datasheet

7
LT1961
1961fa
APPLICATIONS INFORMATION
WUU
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FB RESISTOR NETWORK
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 V
OUT
to FB is:
R
RV
RA
OUT
1
212
12 202
=
−
()
−μ
.
.(.)
defines the pole frequency of the output stage, an X7R or
X5R type ceramic, which have good temperature stability,
is recommended.
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.
Table 1. Surface Mount Solid Tantalum Capacitor ESR and
Ripple Current
E Case Size ESR (Max, Ω ) Ripple Current (A)
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
AVX TAJ 0.7 to 0.9 0.4
D Case Size
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
C Case Size
AVX TPS 0.2 (typ) 0.5 (typ)
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:
I
VV V
LfV
RIPPLE RMS
IN OUT IN
OUT
()
=
()
−
()
()()( )
029.
At higher switching frequency, the energy storage require-
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.
Figure 2. Feedback Network
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:
IIVVV
RIPPLE RMS
OUT OUT IN IN
()
= −
()
/
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
output ripple voltage. Their low ESR removes a useful zero
in the loop frequency response, common to tantalum
capacitors. To compensate for this, the V
C
loop compen-
sation pole frequency must typically be reduced by a factor
of 10. Typical ceramic output capacitors are in the 1μF to
10μF range. Since the absolute value of capacitance
–
+
1.2V
V
SW
V
C
GND
1961 F02
R1
R2
10k
OUTPUT
ERROR
AMPLIFIER
FB
LT1961
+