Datasheet

LTC3823
12
3823fd
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
P
TOP
= D
TOP
I
OUT(MAX)
2
ρ
T(TOP)
R
DS(ON)(MAX)
+ k V
IN
2
I
OUT(MAX)
C
RSS
f
P
BOT
= D
BOT
I
OUT(MAX)
2
ρ
T(BOT)
R
DS(ON)(MAX)
Both MOSFETs have I
2
R losses and the top MOSFET in-
cludes an additional term for transition losses, which are
largest at high input voltages. The constant k = 1.7A
–1
can be
used to estimate the amount of transition loss. The bottom
MOSFET losses are greatest when the bottom duty cycle is
near 100%, during a short-circuit or at high input voltage.
Operating Frequency
The choice of operating frequency is a tradeoff between
effi ciency and component size. Low frequency operation
improves effi ciency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3823 applications is de-
termined implicitly by the one-shot timer that controls
the on-time, t
ON
, of the top MOSFET switch. The on-time
is set by the current out of the I
ON
pin and the voltage at
the V
ON
pin according to:
t
V
I
pF
ON
VON
ION
= ()10
Tying a resistor R
ON
to SGND from the I
ON
pin yields an
on-time inversely proportional to 1/3 V
IN
. The current out
of the I
ON
pin is:
I
V
R
ION
IN
ON
=
3
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
f
V
VRpF
H
OUT
VON ON
Z
=
•()
[]
310
To hold frequency constant during output voltage changes,
tie the V
ON
pin to V
OUT
. The V
ON
pin has internal clamps
that limit its input to the one-shot timer. If the pin is tied
below 0.6V, the input to the one-shot is clamped at 0.6V.
Similarly, if the pin is tied above 4.8V, the input is clamped
at 4.8V. In high V
OUT
applications, tie V
ON
to INTV
CC
. Figures
2a and 2b show how R
ON
relates to switching frequency
for several common output voltages.
JUNCTION TEMPERATURE (°C)
–50
R
T
NORMALIZED ON-RESISTANCE
1.0
1.5
150
3823 F01
0.5
0
0
50
100
2.0
Figure 1. R
DS(ON)
vs Temperature
APPLICATIONS INFORMATION
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000
3823 F02a
V
OUT
= 3.3V
V
OUT
= 1.5V
V
OUT
= 2.5V
R
ON
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100 1000
3823 F02b
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V
Figure 2a. Switching Frequency vs R
ON
(V
ON
= 0V) Figure 2b. Switching Frequency vs R
ON
(V
ON
= INTV
CC
)