LTC1439 Linear Technology, LTC1439 Datasheet - Page 14

• Manufacturer Part Number: LTC1439
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
The peak-to-peak drive levels are set by the INTV
age. This voltage is typically 5V during start-up (see
EXTV
old MOSFETs must be used in most LTC1438/LTC1439
applications. The only exception is applications in which
EXTV
8V (must be less than 10V), in which standard threshold
MOSFETs (V
to the BV
of the logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the "ON"
resistance R
input voltage and maximum output current. When the
LTC1438/LTC1439 are operating in continuous mode the
duty cycles for the top and bottom MOSFETs are given by:
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
LTC1438/LTC1439
14
IN
DS(ON)
P
P
Main Switch Duty Cycle
Synchronous Switch Duty Cycle
< 20V the high current efficiency generally improves
SYNC
MAIN
CC
CC
Pin Connection). Consequently, logic level thresh-
is powered from an external supply greater than
device with lower C
DSS
k V
GS(TH)
V
V
specification for the MOSFETs as well; many
SD(ON)
V
IN
OUT
IN
IN
1.85
V
–
IN
V
< 4V) may be used. Pay close attention
I
, reverse transfer capacitance C
U
OUT
MAX
I
MAX
INFORMATION
I
2
2
MAX
U
R losses while the topside
1
C
RSS
RSS
2
V
V
OUT
1
R
IN
actual provides higher
IN
DS ON
W
f
> 20V the transition
(
R
)
DS ON
V
IN
(
DS(ON)
V
–
IN
U
)
V
OUT
CC
and k
volt-
RSS
,
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
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
used 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.
IN
IN
= 0.005/ C can be used as an approximation for low
OUT
C
. To prevent large voltage transients, a low ESR input
and C
IN
/2. This simple worst-case condition is commonly
Required I
OUT
Selection
RMS
RSS
DS(ON)
is usually specified in the MOSFET
I
MAX
vs Temperature curve, but
V
OUT
IN
= 2V
V
IN
OUT
V
–
IN
, where I
V
OUT
/ 1 2
OUT
RMS
/