ltc3608ewkg Linear Technology Corporation, ltc3608ewkg Datasheet - Page 11

• Manufacturer Part Number: ltc3608ewkg
• Description: 18v, 8a Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The basic LTC3608 application circuit is shown on the
front page of this data sheet. External component selection
is primarily determined by the maximum load current.
The LTC3608 uses the on-resistance of the synchronous
power MOSFET for determining the inductor current. The
desired amount of ripple current and operating frequency
also determines the inductor value. Finally, C
for its ability to handle the large RMS current into the
converter and C
the output voltage ripple and transient specifi cation.
V
The LTC3608 has an open-drain PGOOD output that
indicates when the output voltage is within ±10% of the
regulation point. The LTC3608 also has a V
allows the on-time to be adjusted. Tying the V
results in lower values for R
applications. The V
the on-time to maintain constant frequency operation in
applications where V
frequency shifts with changes in load current.
V
The V
valley current, which in turn determines the maximum
average output current that the LTC3608 can deliver. The
maximum output current is given by:
The I
Current Limit vs V
Characteristics.
An external resistor divider from INTV
set the voltage on the V
be simply tied to ground force a default value equivalent
to 0.7V. When setting current limit ensure that the junc-
tion temperature does not exceed the maximum rating of
125°C. Do not fl oat the V
ON
RNG
I
OUT(MAX)
and PGOOD
VALLEY(MAX)
Pin and I
RNG
pin is used to adjust the maximum inductor
= I
LIMIT
OUT
VALLEY(MAX)
is shown in the fi gure “Maximum Valley
RNG
ON
is chosen with low enough ESR to meet
Adjust
OUT
Voltage” in the Typical Performance
pin also provides a means to adjust
RNG
RNG
changes and to correct minor
ON
+ 1/2 ΔI
pin from 0.5V to 1V, or it can
pin.
which is useful in high V
L
,
CC
can be used to
IN
ON
ON
is selected
pin high
pin that
OUT
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 LTC3608 applications is de-
termined implicitly by the one-shot timer that controls the
on-time t
by the current into the I
pin according to:
Tying a resistor R
on-time inversely proportional to V
the I
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
To hold frequency constant during output voltage changes,
tie the V
when V
limit its input to the one-shot timer. If the pin is tied below
0.7V, the input to the one-shot is clamped at 0.7V. Similarly,
if the pin is tied above 2.4V, the input is clamped at 2.4V.
In high V
comparator input is 2.4V results in a lower value for R
Figures 1a and 1b show how R
frequency for several common output voltages.
I
t
f =
ON
ON
ON
V
=
=
OUT
pin is
VON
ON
R
OUT
ON
V
V
I
ON
ION
IN
VON
pin to V
> 2.4V. The V
of the top MOSFET switch. The on-time is set
V
R
applications, tying V
OUT
ON
(10pF)
(10pF)
ON
OUT
from V
or to a resistive divider from V
ON
[H
ON
pin and the voltage at the V
Z
]
pin has internal clamps that
IN
to the I
ON
ON
IN
to INTV
relates to switching
. The current out of
LTC3608
ON
pin yields an
CC
so that the
11
3608fa
OUT
ON
ON
.