LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
APPLICATIONS INFORMATION
FB RESISTOR NETWORK
If an output voltage of 1.8V, 2.5V, 3.3V or 5V is required,
the respective fixed option part, -1.8, -2.5, -3.3 or -5,
should be used. The FB pin is tied directly to the output;
the necessary resistive divider is already included on
the part. For other voltage outputs, the adjustable part
should be used and an external resistor divider added.
The suggested resistor (R2) from FB to ground is 10k.
This reduces the contribution of FB input bias current to
output voltage to less than 0.25%. The formula for the
resistor (R1) from V
OUT
to FB is:
R1
=
R2( V
OUT
– 1.2)
1.2 – R2(0.25
μ
A)
V
SW
OUTPUT
rating and turn-on surge problems. Y5V or similar type
ceramics can be used since the absolute value of capaci-
tance is less important and has no significant effect on
loop stability. If operation is required close to the minimum
input required by the output or the LT1765, a larger value
may be required. This is to prevent excessive ripple caus-
ing dips below the minimum operating voltage resulting
in erratic operation.
If tantalum capacitors are used, values in the 22μF to 470μF
range are generally needed to minimize ESR and meet
ripple current and surge ratings. Care should be taken to
ensure the ripple and surge ratings are not exceeded. The
AVX TPS and Kemet T495 series tantalum capacitors are
surge rated. AVX recommends derating capacitor operating
voltage by 2:1 for high surge applications.
OUTPUT CAPACITOR
Unlike the input capacitor, RMS ripple current in the output
capacitor is normally low enough that ripple current rating
is not an issue. The current waveform is triangular, with
an RMS value given by:
LT1765 (ADJ)
ERROR
AMPLIFIER
+
–
1.2V
FB
R1
+
R2
10k
1765 F02
V
C
GND
I
RIPPLE
(
RMS
)
=
0.29
(
V
OUT
) (
V
IN
−
V
OUT
)
Figure 2. Feedback Network
(
L
)(
f
)
(
V
IN
)
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the voltage
ripple at the input of LT1765 and to force the switching
current into a tight local loop, thereby minimizing EMI.
The RMS ripple current can be calculated from:
I
RIPPLE
(
RMS
)
=
I
OUT
V
OUT
(
V
IN
−
V
OUT
)
/
V
IN2
Ceramic capacitors are ideal for input bypassing. At higher
switching frequency, the energy storage requirement of
the input capacitor is reduced so values in the range of
1μF to 4.7μF are suitable for most applications. Their high
frequency capacitive nature removes most ripple current
The LT1765 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. Ceramic
output capacitors in the 1μF to 10μF range, X7R or X5R
type are recommended.
Tantalum capacitors are usually chosen for their bulk
capacitance properties, useful in high transient load ap-
plications. ESR rather than absolute value defines output
ripple at 1.25MHz. Typical LT1765 applications require a
tantalum capacitor with less than 0.3Ω ESR at 22μF to
500μF see Table 2. This ESR provides a useful zero in the
,
frequency response. Ceramic output capacitors with low
ESR usually require a larger V
C
capacitor or an additional
series R to compensate for this.
1765fd
8
LINER [ LINEAR TECHNOLOGY ]
