lt1765es8-trpbf Linear Technology Corporation, lt1765es8-trpbf Datasheet - Page 8

• Manufacturer Part Number: lt1765es8-trpbf
• Description: Monolithic 3a, 1.25mhz Step-down Switching Regulators
• Manufacturer: Linear Technology Corporation

LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
APPLICATIO S I FOR ATIO
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 sug-
gested 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
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:
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
8
R
I
RIPPLE RMS
1
=
V
LT1765 (ADJ)
1 2
C
R V
. –
( )
2
GND
OUT
(
AMPLIFIER
R
OUT
ERROR
2 0 25
to FB is:
Figure 2. Feedback Network
( .
=
I
– . )
OUT
U
1 2
µ
+
–
A
V
)
U
OUT
1.2V
(
V
V
IN
SW
FB
−
W
V
OUT
R1
R2
10k
+
)
/
1765 F02
V
OUTPUT
IN
U
2
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 mini-
mum input required by the output or the LT1765, a larger
value may be required. This is to prevent excessive ripple
causing dips below the minimum operating voltage result-
ing 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 operat-
ing 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 cur-
rent rating is not an issue. The current waveform is
triangular, with an RMS value given by:
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 appli-
cations. 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
series R to compensate for this.
I
RIPPLE RMS
(
)
=
0 29
.
(
V
OUT
( )( )( )
L f V
)
C
(
V
capacitor or an additional
IN
IN
−
V
OUT
)
1765fb