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LTC3608IWKG#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LTC3608
10
3608fc
operation
Main Control Loop
The LTC3608 is a high efficiency monolithic synchronous,
step-down DC/DC converter utilizing a constant on-time,
current mode architecture. It operates from an input volt-
age range of 4V to 18V (20V maximum) and provides a
regulated output voltage at up to 8A of output current. The
internal synchronous power switch increases efficiency
and eliminates the need for an external Schottky diode. In
normal operation, the top MOSFET is turned on for a fixed
interval determined by a one-shot timer OST. When the
top MOSFET is turned off, the bottom MOSFET is turned
on until the current comparator I
CMP
trips, restarting the
one-shot timer and initiating the next cycle. Inductor current
is determined by sensing the voltage between the PGND
and SW pins using the bottom MOSFET on-resistance.
The voltage on the I
TH
pin sets the comparator threshold
corresponding to inductor valley current. The error ampli-
fier, EA, adjusts this voltage by comparing the feedback
signal V
FB
from the output voltage with an internal 0.6V
reference. If the load current increases, it causes a drop
in the feedback voltage relative to the reference. The I
TH
voltage then rises until the average inductor current again
matches the load current.
At light load, the inductor current can drop to zero and
become negative. This is detected by current reversal
comparator I
REV
which then shuts off M2 (see Func-
tional Diagram), resulting in discontinuous operation. Both
switches will remain off with the output capacitor supplying
the load current until the I
TH
voltage rises above the zero
current level (0.8V) to initiate another cycle. Discontinu-
ous mode operation is disabled by comparator F when
the FCB pin is brought below 0.6V, forcing continuous
synchronous operation.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to main-
tain regulation. The one-shot timer generates an on-time
that is proportional to the ideal duty cycle, thus holding
frequency approximately constant with changes in V
IN
.
The nominal frequency can be adjusted with an external
resistor, R
ON
.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback volt-
age exits a ±10% window around the regulation point.
Furthermore, in an overvoltage condition, M1 is turned
off and M2 is turned on and held on until the overvoltage
condition clears.
Foldback current limiting is provided if the output is
shorted to ground. As V
FB
drops, the buffered current
threshold voltage I
THB
is pulled down by clamp Q3 to
a 1V level set by Q4 and Q6. This reduces the inductor
valley current level to one sixth of its maximum value as
V
FB
approaches 0V.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2µA current source to charge
up an external soft-start capacitor, C
SS
. When this voltage
reaches 1.5V, the controller turns on and begins switching,
but with the I
TH
voltage clamped at approximately 0.6V
below the RUN/SS voltage. As C
SS
continues to charge,
the soft-start current limit is removed.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most of
the internal controller circuitry is derived from the INTV
CC
pin. The top MOSFET driver is powered from a floating
bootstrap capacitor
,
C
B
. This capacitor is recharged from
INTV
CC
through an external Schottky diode
,
D
B
,
when
the top MOSFET is turned off. When the EXTV
CC
pin is
grounded, an internal 5V low dropout regulator supplies
the INTV
CC
power from V
IN
. If EXTV
CC
rises above 4.7V,
the internal regulator is turned off, and an internal switch
connects EXTV
CC
to INTV
CC
. This allows a high efficiency
source connected to EXTV
CC
, such as an external 5V sup-
ply or a secondary output from the converter, to provide
the INTV
CC
power. Voltages up to 7V can be applied to
EXTV
CC
for additional gate drive. If the input voltage is
low and INTV
CC
drops below 3.5V, undervoltage lockout
circuitry prevents the power switches from turning on.
LTC3608
11
3608fc
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
IN
is selected
for its ability to handle the large RMS current into the
converter and C
OUT
is chosen with low enough ESR to meet
the output voltage ripple and transient specification.
V
ON
and PGOOD
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
ON
pin that
allows the on-time to be adjusted. Tying the V
ON
pin high
results in lower values for R
ON
which is useful in high V
OUT
applications. The V
ON
pin also provides a means to adjust
the on-time to maintain constant frequency operation in
applications where V
OUT
changes and to correct minor
frequency shifts with changes in load current.
V
RNG
Pin and I
LIMIT
Adjust
The V
RNG
pin is used to adjust the maximum inductor
valley current, which in turn determines the maximum
average output current that the LTC3608 can deliver. The
maximum output current is given by:
I
OUT(MAX)
= I
VALLEY(MAX)
+ 1/2 ΔI
L
The I
VALLEY(MAX)
is shown in the figure “Maximum Valley
Current Limit vs V
RNG
Voltage” in the Typical Performance
Characteristics.
An external resistor divider from INTV
CC
can be used to
set the voltage on the V
RNG
pin from 0.5V to 1V, or it can
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 float the V
RNG
pin.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency 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
ON
, of the top MOSFET switch. The on-time is
set by the current into the I
ON
pin and the voltage at the
V
ON
pin according to:
t
ON
=
V
VON
I
ION
(10pF)
Tying a resistor R
ON
from V
IN
to the I
ON
pin yields an
on-time inversely proportional to V
IN
. The current out of
the I
ON
pin is
I
ON
=
V
IN
R
ON
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
f =
V
OUT
V
VON
R
ON
(10pF)
[H
Z
]
To hold frequency constant during output voltage changes,
tie the V
ON
pin to V
OUT
or to a resistive divider from V
OUT
when V
OUT
> 2.4V. The V
ON
pin has internal clamps that
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
OUT
applications, tying V
ON
to INTV
CC
so that the
comparator input is 2.4V results in a lower value for R
ON
.
Figures 1a and 1b show how R
ON
relates to switching
frequency for several common output voltages.
LTC3608
12
3608fc
applications inForMation
Figure 1a. Switching Frequency vs R
ON
(V
ON
= 0V)
Figure 1b. Switching Frequency vs R
ON
(V
ON
= INTV
CC
)
Because the voltage at the I
ON
pin is about 0.7V, the cur-
rent into this pin is not exactly inversely proportional to
V
IN
, especially in applications with lower input voltages.
To correct for this error, an additional resistor, R
ON2
,
connected from the I
ON
pin to the 5V INTV
CC
supply will
further stabilize the frequency.
R
ON2
=
5V
0.7V
R
ON
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I
TH
pin to the V
ON
pin and V
OUT
. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the I
TH
pin to the V
ON
pin
as shown in Figure 2a. Place capacitance on the V
ON
pin
to filter out the I
TH
variations at the switching frequency.
The resistor load on I
TH
reduces the DC gain of the error
amp and degrades load regulation, which can be avoided
by using the PNP emitter follower of Figure 2b.
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3608 F01a
V
OUT
= 3.3V
V
OUT
= 1.5V
V
OUT
= 2.5V
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3608 F01b
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26

LTC3608IWKG#PBF

Mfr. #:
Buy LTC3608IWKG#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 8A (Iout) 18Vin (20V Max.) Synch Step Down Reg
Lifecycle:
New from this manufacturer.
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