LTC1538-AUX Linear Technology, LTC1538-AUX Datasheet - Page 14

• Manufacturer Part Number: LTC1538-AUX
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
The MOSFET power dissipations at maximum output
current are given by:
where is the temperature dependency of R
is a constant inversely related to the gate drive current.
Both MOSFETs have I
N-channel equation includes an additional term for transi-
tion losses, which are highest at high input voltages. For
V
with larger MOSFETs, while for V
losses rapidly increase to the point that the use of a higher
R
efficiency. The synchronous MOSFET losses are greatest
at high input voltage or during a short circuit when the duty
cycle in this switch is nearly 100%. Refer to the Foldback
Current Limiting section for further applications information.
The term (1 + ) is generally given for a MOSFET in the
form of a normalized R
voltage MOSFETs. C
characteristics. The constant k = 2.5 can be used to
estimate the contributions of the two terms in the main
switch dissipation equation.
The Schottky diode D1 shown in Figure 1 serves two
purposes. During continuous synchronous operation, D1
conducts during the dead-time between the conduction of
the two large power MOSFETs. This prevents the body
diode of the bottom MOSFET from turning on and storing
charge during the dead-time, which could cost as much as
1% in efficiency. During low current operation, D1 oper-
ates in conjunction with the small top MOSFET to provide
an efficient low current output stage. A 1A Schottky is
LTC1538-AUX/LTC1539
14
IN
DS(ON)
= 0.005/ C can be used as an approximation for low
P
P
< 20V the high current efficiency generally improves
SYNC
MAIN
device with lower C
k V
V
V
V
IN
OUT
IN
IN
1.85
V
–
IN
V
I
U
OUT
MAX
RSS
I
MAX
DS(ON)
is usually specified in the MOSFET
INFORMATION
I
2
2
MAX
U
R losses while the topside
1
C
RSS
vs Temperature curve, but
RSS
2
1
R
actual provides higher
IN
DS ON
W
f
> 20V the transition
(
R
)
DS ON
(
DS(ON)
U
)
and k
generally a good compromise for both regions of opera-
tion due to the relatively small average current.
C
In continuous mode, the source current of the top
N-channel MOSFET is a square wave of duty cycle V
V
capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
This formula has a maximum at V
I
for design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple current
ratings are often based on only 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 also be paralleled to meet size or height
requirements in the design. Always consult the manufacturer
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
where f = operating frequency, C
and I
is highest at maximum input voltage since I
with input voltage. With I
ripple will be less than 100mV at max V
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
OUT
IN
IN
C
C Required I
. To prevent large voltage transients, a low ESR input
and C
/2. This simple worst-case condition is commonly used
OUT
IN
V
OUT
L
= ripple current in the inductor. The output ripple
Required ESR < 2R
OUT
Selection
I ESR
L
RMS
OUT
OUT
is driven by the required effective
) is approximated by:
4
I
MAX
fC
SENSE
L
1
OUT
= 0.4I
V
OUT
IN
OUT
= 2V
= output capacitance
OUT(MAX)
V
OUT
IN
IN
V
–
assuming:
IN
, where I
V
L
OUT
the output
increases
RMS
/ 1 2
OUT
=
/