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LTC3613IWKH#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36
LTC3613
10
3613fa
FUNCTIONAL DIAGRAM
–
+
–
+
I
CMP
EA
(g
m(EA)
= 1.7mS)
0.645V
0.555V
0.6V
1µA
I
REV
LOGIC
CONTROL
PLL
SYSTEM
ONE-SHOT
TIMER
CLOCK
DETECT
OSCILLATOR
LDO
OUT EN
IN
SV
IN
4.2V
TIME
ADJUST
STOP
START
1.2V
0.75V
–
+
+
–
+
DA
(A = 1)
R
PGD
PGOOD
MODE/PLLIN
V
RNG
ITH
V
OSNS
–
V
OSNS
+
TRACK/SS
SENSE
–
SENSE
+
PGND
SW
EXTV
CC
BO0ST
INPUT SUPPLY
D
B
C
B
MT
MB
L
C
OUT
R
FB2
R
FB1
R
SENSE
PV
IN
INTV
CC
INTV
CC
SGND
R
ITH
C
ITH1
INTV
CC
R1
INTV
CC
R2
3.65V
UVLO
4.6V
V
OUT
RT
RT
RUN
1.3µA
C
VCC
V
IN
C
IN
V
OUT
C
IN
C
SS
3613 FD
+
–
UV
+
–
+
–
–
+
–
–
OV
+
–
TG DRV
BG DRV
CLOCK
OPERATION
Main Control Loop
The LTC3613 uses valley current mode control to regulate
the output voltage in a monolithic, all N-channel MOSFET
DC/DC step-down converter. Current control is achieved by
sensing the inductor current across SENSE
+
and SENSE
–
,
either by using an explicit resistor connected in series with
the inductor or by implicitly sensing the inductor’s resis-
tive (DCR) voltage drop through an RC filter connected
across the inductor.
In normal steady-state operation, the top MOSFET is turned
on for a fixed time interval proportional to the delay in the
one-shot timer. The PLL system adjusts the delay in the
one-shot timer until the top MOSFET turn-on is synchro-
nized either to the internal oscillator or the external clock
input if provided. As the top MOSFET turns off, the bottom
MOSFET turns on with a small time delay (dead time) to
avoid shoot-through current. The next switching cycle is
initiated when the current comparator, I
CMP
, senses that
inductor current has reached the valley threshold point
(Refer to Functional Diagram)
LTC3613
11
3613fa
OPERATION
(Refer to Functional Diagram)
and turns the bottom MOSFET off immediately and the
top MOSFET on. Again in order to avoid shoot-through
current there is a small dead time delay before the top
MOSFET turns on.
The voltage on the ITH pin sets the I
CMP
valley threshold
point. The error amplifier, EA, adjusts this ITH voltage
by comparing the differential feedback signal, V
OSNS
+
−
V
OSNS
–
, to a 0.6V internal reference voltage. Consequently,
the LTC3613 regulates the output voltage by forcing the
differential feedback voltage to be equal to the 0.6V internal
reference. The difference amplifier, DA, converts the dif-
ferential feedback signal to a single-ended input for the
EA. If the load current increases, it causes a drop in the
differential feedback voltage relative to the reference. The
EA forces ITH voltage to rise until the average inductor
current again matches the load current.
Differential Output Sensing
The output voltage is resistively divided externally to create
a feedback voltage for the controller. The internal difference
amplifier, DA, senses this feedback voltage along with the
output’s remote ground reference to create a differential
feedback voltage. This scheme overcomes any ground
offsets between local ground and remote output ground,
resulting in a more accurate output voltage. The LTC3613
allows for remote output ground deviations as much as
±500mV with respect to local ground.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
CC
pin. Power
on the INTV
CC
pin is derived in two ways: if the EXTV
CC
pin is below 4.6V, then an internal 5.3V low dropout linear
regulator, LDO, supplies INTV
CC
power from PV
IN
; if the
EXTV
CC
pin is tied to an external source larger than 4.6V,
then the LDO is shut down and an internal switch shorts
the EXTV
CC
pin to the INTV
CC
pin, thereby powering the
INTV
CC
pin with the external source and helping to increase
overall efficiency and decrease internal self heating through
power dissipated in the LDO. This external power source
could be the output of the step-down switching regulator
itself if the output is programmed to higher than 4.6V.
The top MOSFET driver is biased from the floating boot-
strap capacitor, C
B
, which normally recharges during
each off cycle through an external Schottky diode when
the top MOSFET turns off. If the V
IN
voltage is low and
INTV
CC
drops below 3.65V, undervoltage lockout circuitry
disables the external MOSFET driver and prevents the
power switches from turning on.
Shutdown and Start-Up
The LTC3613 can be shut down using the RUN pin. Pull-
ing this pin below 1.2V prevents switching, and less than
0.75V disables most of the internal bias circuitry, including
the INTV
CC
regulator. When RUN is less than 0.75V, the
shutdown I
Q
is about 15A. Pulling the RUN pin between
0.75V and 1.2V enables the controller into a standby mode
where all internal circuitry is powered-up except for the
MOSFET driver. The standby I
Q
is about 2mA. Releasing
the RUN pin from ground allows an internal 1.3A current
to pull the pin above 1.2V and fully enable the controller
including the MOSFET driver. Alternatively, the RUN pin
may be externally pulled up or driven directly by logic. Be
careful not to exceed the absolute maximum rating of 6V
on this pin. When pulled up by a resistor to an external
voltage, the RUN pin will sink about 35µA of current before
reaching 6V. If the external voltage is above 6V (e.g., V
IN
),
select a large enough resistor value so that the voltage on
RUN will not exceed 6V.
The start-up of the output voltage, V
OUT
, is controlled by
the voltage on the TRACK/SS pin. When the voltage on
the TRACK/SS pin is less than the 0.6V internal reference,
the LTC3613 regulates the differential feedback voltage to
the TRACK/SS voltage instead of the 0.6V reference. This
allows the TRACK/SS pin to be used for programming a
ramp-up time for V
OUT
by connecting an external capacitor
from the TRACK/SS pin to SGND. An internal 1A pull-up
current charges this capacitor, creating a voltage ramp on
the TRACK/SS pin. As the TRACK/SS voltage rises from
0V to 0.6V (and beyond), the LTC3613 forces the output
voltage, V
OUT
, to ramp up smoothly to its final value.
Alternatively, the TRACK/SS pin can be used to track the
start-up of V
OUT
to another external supply as in a master
slave configuration. Typically, this requires connecting a
resistor divider from the master supply to the TRACK/SS
pin (see Soft-Start and Tracking).
LTC3613
12
3613fa
OPERATION
When the RUN pin is pulled low to disable the controller or
when INTV
CC
drops below its undervoltage lockout thresh-
old of 3.65V, the TRACK/SS pin is pulled low internally.
Light Load Current Operation
When the DC load current is less than 1/2 of the peak-
to-peak inductor current ripple, the inductor current can
drop to zero or become negative. If the MODE/PLLIN pin
is connected to SGND, the LTC3613 will transition into
discontinuous mode operation (also called pulse-skipping
mode), where a current reversal comparator, I
REV
, detects
and prevents negative inductor current by shutting off the
bottom MOSFET, MB. In this mode, both switches remain
off with the output capacitor supplying the load current.
As the output capacitor discharges and the output volt-
age droops lower, the EA will eventually move the ITH
voltage above the zero current level to initiate another
switching cycle.
If the MODE/PLLIN pin is tied to INTV
CC
or an external
clock is applied to MODE/PLLIN, the LTC3613 will be forced
to operate in continuous mode (forced continuous mode)
and not transition into discontinuous mode. In this case
the current reversal comparator, I
REV
, is disabled, allowing
the inductor current to become negative and thus maintain
constant frequency operation.
Frequency Selection and External Clock
Synchronization
The steady-state switching frequency of the LTC3613 is
set by an internal oscillator. The frequency of this internal
oscillator can be programmed from 200kHz to 1MHz by
connecting a resistor from the RT pin to SGND. The RT
pin is forced to 1.2V internally. A phase-locked loop (PLL)
system synchronizes the turn-on of the switching cycle to
this internal oscillator when no external clock is provided.
For applications with stringent frequency or interfer-
ence requirements, an external clock source connected
to the MODE/PLLIN pin can be used to synchronize the
switching cycle turn-on to the rising edge of the clock.
The LTC3613 operates in forced continuous mode when
it is synchronized to the external clock. The external clock
frequency has to be within ±30% of the internal oscillator
frequency for successful synchronization and the clock
input levels should be greater than 2V for HI and less
than 0.5V for LO. The MODE/PLLIN pin has an internal
600k pull-down resistor.
Power Good and Fault Protection
The power good pin, PGOOD, is connected internally to an
open-drain N-channel MOSFET. An external pull-up resistor
to a voltage supply of up to 6V (or INTV
CC
) completes the
power good detection scheme. Overvoltage and undervolt-
age comparators OV and UV turn on the MOSFET and pull
the PGOOD pin low when the differential feedback voltage
is outside a ±7.5% window of the 0.6V reference voltage.
The PGOOD pin is also pulled low when the LTC3613 is
in the soft-start or tracking phase, when in undervoltage
lockout, or when the RUN pin is low (shut down).
When the differential feedback voltage is within the ±7.5%
requirement, the open-drain NMOS is turned off and the
pin is pulled up by an external resistor. There is an internal
delay of 10µs before the PGOOD pin will indicate power
good once the differential feedback voltage is within the
±7.5% window. When the feedback voltage goes out of
the ±7.5% window, there is an internal 20s delay before
PGOOD is pulled low. In an overvoltage condition, MT is
turned off and MB is turned on immediately without any
delay and held on until the overvoltage condition clears.
Foldback current limiting is provided if the output is shorted
to ground. As the differential feedback voltage drops, the
current threshold voltage on the ITH pin is pulled down
and clamped to 1.2V. This reduces the inductor valley
current level to one-fourth of its maximum value as the
differential feedback approaches 0V. Foldback current
limiting is disabled at start-up.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36

LTC3613IWKH#TRPBF

Mfr. #:
Buy LTC3613IWKH#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Fast, Accurate, Monolithic Step-Down Regulator with Differential Output Sensing
Lifecycle:
New from this manufacturer.
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