Datasheet

LTC1772B
9
1772bfa
APPLICATIONS INFORMATION
Since the LTC1772B is designed for operation down to low
input voltages, a logic level threshold MOSFET (R
DS(ON)
guaranteed at V
GS
= 2.5V) is required for applications that
work close to this voltage. When these MOSFETs are used,
make sure that the input supply to the LTC1772B is less
than the absolute maximum V
GS
rating, typically 8V.
The required minimum R
DS(ON)
of the MOSFET is governed
by its allowable power dissipation. For applications that
may operate the LTC1772B in dropout, i.e., 100% duty
cycle, at its worst case the required R
DS(ON)
is given by:
R
DS(ON)
DC=100%
=
P
P
I
OUT(MAX)
()
2
1+p
( )
where P
P
is the allowable power dissipation and δp is the
temperature dependency of R
DS(ON)
. (1 + δp) is generally
given for a MOSFET in the form of a normalized R
DS(ON)
vs temperature curve, but δp = 0.005/°C can be used as
an approximation for low voltage MOSFETs.
In applications where the maximum duty cycle is less
than 100% and the LTC1772B is in continuous mode, the
R
DS(ON)
is governed by:
R
DS(ON)

P
P
DC
()
I
OUT
2
1+p
( )
where DC is the maximum operating duty cycle of the
LTC1772B.
Output Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As V
IN
approaches V
OUT
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short-circuited. Under this condition the diode must
safely handle I
PEAK
at close to 100% duty cycle. Therefore,
it is important to adequately specify the diode peak cur-
rent and average power dissipation so as not to exceed
the diode ratings.
Under normal load conditions, the average current con-
ducted by the diode is:
I
D
=
V
IN
 V
OUT
V
IN
+ V
D






I
OUT
The allowable forward voltage drop in the diode is calculated
from the maximum short-circuit current as:
V
P
I
F
D
SC MAX
≈
()
where P
D
is the allowable power dissipation and will be
determined by effi ciency and/or thermal requirements.
A fast switching diode must also be used to optimize
effi ciency. 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
and C
OUT
Selection
In continuous mode, the source current of the P-chan-
nel MOSFET is a square wave of duty cycle (V
OUT
+ V
D
)/
(V
IN
+ V
D
). To prevent large voltage transients, a low
ESR input capacitor sized for the maximum RMS current
must be used. The maximum RMS capacitor current is
given by:
CI
VVV
V
IN MAX
OUT IN OUT
IN
Required I
RMS
≈
−
()
[]
12/
This formula has a maximum value at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even signifi cant 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