LTC1779 LINER [Linear Technology], LTC1779 Datasheet - Page 8

• Manufacturer Part Number: LTC1779
• Description: 250mA Current Mode Step-Down DC/DC Converter in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1779
Under normal load conditions, the average current con-
ducted by the diode is:
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
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 internal
P-channel MOSFET is a square wave of duty cycle
(V
sients, a low ESR input capacitor sized for the maximum
RMS current must be used. The maximum RMS capaci-
tor current is given by:
This formula has a maximum at V
= I
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 LTC1779, ceramic capacitors
can also be used for C
if there is any question.
8
IN
OUT
OUT
I
C
V
D
and C
F
IN
/2. This simple worst-case condition is commonly
+ V
Required I
I
D
CL MAX
V
OUT
is the allowable power dissipation and will be
V
D
IN
P
(
)/(V
IN
D
Selection
V
IN
)
OUT
V
D
RMS
+ V
U
IN
I
D
OUT
). To prevent large voltage tran-
. Always consult the manufacturer
I
U
MAX
V
OUT
IN
W
= 2V
V
IN
V
OUT
IN
V
, where I
OUT
U
1 2 /
RMS
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
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
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS, AVX TPSV and KEMET T510 series of surface mount
tantalum, available in case heights ranging from 2mm to
4mm. Other capacitor types include Sanyo OS-CON,
Nichicon PL series and Panasonic SP.
Low Supply Operation
Although the LTC1779 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)
V
OUT
L
increases with input voltage.
requirement.
I
RIPPLE
IN
RIPPLE
decreases below 3V. Figure 3 shows the
OUT
ESR
OUT
is driven by the required effective
is the ripple current in the induc-
) is approximated by:
8
fC
1
OUT
OUT
IN
on V
is the output
OUT
REF
has been
as V
IN