ltc1772b Linear Technology Corporation, ltc1772b Datasheet - Page 8

• Manufacturer Part Number: ltc1772b
• Description: Constant Frequency Current Mode Step-down Dc/dc Controller In Sot-23
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC1772B
where P
determined by efficiency and/or thermal requirements.
A fast switching diode must also be used to optimize
efficiency. Schottky diodes are a good choice for low
forward drop and fast switching times. Remember to keep
lead length short and observe proper grounding (see
Board Layout Checklist) to avoid ringing and increased
dissipation.
C
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (V
(V
input capacitor sized for the maximum RMS current must
be used. The maximum RMS capacitor current is given by:
This formula has a maximum value at V
I
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC1772B, ceramic capacitors
can also be used for C
if there is any question.
The selection of C
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple ( V
8
RMS
IN
IN
C
and C
+ V
IN
V
= I
OUT
Required I
D
OUT
D
). To prevent large voltage transients, a low ESR
OUT
is the allowable power dissipation and will be
/2. This simple worst-case condition is com-
I
RIPPLE
Selection
RMS
OUT
U
ESR
IN
OUT
is driven by the required effective
. Always consult the manufacturer
INFORMATION
I
) is approximated by:
U
MAX
4
fC
V
1
OUT
OUT
W
V
IN
IN
V
= 2V
IN
V
OUT
OUT
OUT
U
, where
+ V
1 2 /
D
)/
where f is the operating frequency, C
capacitance and I
tor. The output ripple is highest at maximum input voltage
since I
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR (size)
product of any aluminum electrolytic at a somewhat
higher price. Once the ESR requirement for C
met, the RMS current rating generally far exceeds the
I
Low Supply Operation
Although the LTC1772B can function down to approxi-
mately 2.0V, the maximum allowable output current is
reduced when V
amount of change as the supply is reduced down to 2V.
Also shown in Figure 4 is the effect of V
goes below 2.3V.
RIPPLE(P-P)
L
increases with input voltage.
Figure 4. Line Regulation of V
requirement.
105
100
95
90
85
80
75
2.0
IN
RIPPLE
decreases below 3V. Figure 4 shows the
2.2
INPUT VOLTAGE (V)
is the ripple current in the induc-
V
V
2.4
REF
ITH
2.6
REF
2.8
OUT
IN
and V
1772 F03
on V
is the output
OUT
3.0
ITH
REF
has been
as V
IN