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LTC3867IUF#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36
LTC3867
25
3867f
APPLICATIONS INFORMATION
the soft-start phase expires, the LTC3867 is forced into
continuous mode of operation as soon as V
FB
is below
the undervoltage threshold of 0.555V regardless of the
setting on the MODE pin. However, the LTC3867 should
always be set in forced continuous mode tracking down
when there is no load. After TK/SS drops below 0.1V, the
controller operates in discontinuous mode.
The LTC3867 allows the user to program how its output
ramps up and down by means of the TK/SS pin. Through
these pins, the output can be set up to either coincidentally
or ratiometrically track another supply’s output, as shown
in Figure 10. In the following discussions, V
OUT2
refers to
the LTC3867’s output as a slave and V
OUT1
refers to another
supply output as a master. To implement the coincident
tracking in Figure 10a, connect an additional resistive di-
vider to V
OUT1
and connect its mid-point to the TK/SS pin
of the slave controller. The ratio of this divider should be
the same as that of the slave controller’s feedback divider
shown in Figure 11a. In this tracking mode, V
OUT1
must
be set higher than V
OUT2
. To implement the ratiometric
tracking in Figure 10b, the ratio of the V
OUT2
divider should
be exactly the same as the master controller’s feedback
divider shown in Figure 11b . By selecting different resis-
tors, the LTC3867 can achieve different modes of tracking
including the two in Figure 10.
So which mode should be programmed? While either
mode in Figure 10 satisfies most practical applications,
some trade-offs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation. Under ratiometric tracking, when the master
controller’s output experiences dynamic excursion (under
load transient, for example), the slave controller output
will be affected as well. For better output regulation, use
the coincident tracking mode instead of ratiometric.
Figure 10. Two Different Modes of Output Voltage Tracking
Figure 11. Setup and Coincident and Ratiometric Tracking
TIME
(10a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
V
OUT1
V
OUT2
TIME
3867 F10
(10b) Ratiometric Tracking
OUTPUT VOLTAGE
R3 R1
R4 R2
R3
V
OUT2
R4
(11a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3867 F11
(11b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
LTC3867
26
3867f
INTV
CC
(LDO) and EXTV
CC
The LTC3867 features a true PMOS LDO that supplies
power to INTV
CC
from the V
IN
supply. INTV
CC
powers the
gate drivers and much of the LTC3867’s internal circuitry.
The LDO regulates the voltage at the INTV
CC
pin to 5.3V
when V
IN
is greater than 5.8V. EXTV
CC
connects to INTV
CC
through a P-channel MOSFET and can supply the needed
power when its voltage is higher than 4.7V. Either of these
can supply a peak current of 100mA and must be bypassed
to ground with a minimum of 4.7µF ceramic capacitor or
low ESR electrolytic capacitor. No matter what type of bulk
capacitor is used, an additional 0.1µF ceramic capacitor
placed directly adjacent to the INTV
CC
and PGND pins is
highly recommended. Good bypassing is needed to sup-
ply the high transient currents required by the MOSFET
gate drivers. High input voltage applications in which
large MOSFETs are being driven at high frequencies may
cause the maximum junction temperature rating for the
LTC3867 to be exceeded. The INTV
CC
current, which is
dominated by the gate charge current, may be supplied by
either the 5.3V LDO or EXTV
CC
. When the voltage on the
EXTV
CC
pin is less than 4.5V, the LDO is enabled. Power
dissipation for the IC in this case is highest and is equal
to V
IN
• I
INTVCC
. The gate charge current is dependent
on operating frequency as discussed in the Efficiency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 2 of the
Electrical Characteristics tables. For example, the LTC3867
INTV
CC
current is limited to less than 30mA from a 38V
supply in the UF package and not using the EXTV
CC
supply
with a 70°C ambient temperature:
T
J
= 70°C + (30mA)(38V)(47°C/W) ≅ 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (MODE = SGND)
at maximum V
IN
. When the voltage applied to EXTV
CC
rises
above 4.7V, the INTV
CC
LDO is turned off and the EXTV
CC
is connected to the INTV
CC
. The EXTV
CC
remains on as
long as the voltage applied to EXTV
CC
remains above 4.5V.
Using the EXTV
CC
allows the MOSFET driver and control
power to be derived from an efficient switching regulator
output during normal operation. If more current is required
through the EXTV
CC
than is specified, an external Schottky
diode can be added between the EXTV
CC
and INTV
CC
pins.
Do not apply more than 6V to the EXTV
CC
pin and make
sure that EXTV
CC
< V
IN
.
Significant efficiency and thermal gains can be realized
by powering INTV
CC
from EXTV
CC
, since the V
IN
current
resulting from the driver and control currents will be scaled
by a factor of (duty cycle)/(switcher efficiency). Tying the
EXTV
CC
pin to a 5V supply reduces the junction temperature
in the previous example from 125°C to:
T
J
= 70°C + (30mA)(5V)(47°C/W) = 77°C
However, for low voltage outputs, additional circuitry is
required to derive INTV
CC
power from the output.
The following list summarizes the four possible connec-
tions for EXTV
CC
:
1. EXTV
CC
left open (or grounded). This will cause
INTV
CC
to be powered from the internal LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV
CC
connected directly to V
OUT
. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV
CC
connected to an external supply. If a 5V external
supply is available, it may be used to power EXTV
CC
providing it is compatible with the MOSFET gate drive
requirements.
4. EXTV
CC
connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.7V.
For applications where the main input power is 5V, tie
the V
IN
and INTV
CC
pins together and tie the combined
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 12 to minimize the voltage drop caused by the
gate charge current. This will override the INTV
CC
linear
regulator and will prevent INTV
CC
from dropping too low
due to the dropout voltage. Make sure the INTV
CC
voltage
is at or exceeds the R
DS(ON)
test voltage for the MOSFET
which is typically 4.5V for logic-level devices
APPLICATIONS INFORMATION
LTC3867
27
3867f
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitor, C
B
, connected to the BOOST
pin supplies the gate drive voltages for the topside MOSFET.
Capacitor C
B
in the Functional Diagram is charged though
external diode D
B
from INTV
CC
when the SW pin is low.
When the topside MOSFET is to be turned on, the driver
places the C
B
voltage across the gate source of the MOSFET.
This enhances the MOSFET and turns on the topside switch.
The switch node voltage, SW, rises to V
IN
and the BOOST
pin follows. With the topside MOSFET on, the boost volt-
age is above the input supply:
V
BOOST
= V
IN
+ V
INTVCC
– V
DB
The value of the boost capacitor, C
B
, needs to be 100 times
that of the total input capacitance of the topside MOSFET(s).
The reverse breakdown of the external Schottky diode
must be greater than V
IN(MAX)
. When adjusting the gate
drive level, the final arbiter is the total input current for
the regulator. If a change is made and the input current
decreases, then the efficiency has improved. If there is
no change in input current, then there is no change in
efficiency.
Setting Output Voltage
The LTC3867 output voltage is set by an external feed-
back resistive divider carefully placed across the output,
as shown in Figure 13. The regulated output voltage is
determined by:
V
OUT
= 0.6V • 1+
R
B
R
A
To improve the frequency response, a feedforward ca-
pacitor, C
FF
, may be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW line.
If the diffamp is used, then the resistor divider center tap
should connect to the noninverting input of the diffamp,
DIFF
+
. DIFFOUT should then be shorted to V
FB
.
Fault Conditions: Current Limit and Current Foldback
The LTC3867 includes current foldback to help limit load
current when the output is shorted to ground. If the out-
put falls below 50% of its nominal output level, then the
maximum sense voltage is progressively lowered from its
maximum programmed value to one-third of the maximum
value. Foldback current limiting is disabled during the soft-
start or tracking up using the TK/SS pin. It is not disabled
for internal soft-start. Under short-circuit conditions with
very low duty cycles, the LTC3867 will begin cycle skipping
in order to limit the short-circuit current. In this situation
the bottom MOSFET will be dissipating most of the power
but less than in normal operation. The short circuit ripple
current is determined by the minimum on-time t
ON(MIN)
of
the LTC3867 (≈65ns), the input voltage and inductor value:
∆I
L(SC)
= t
ON(MIN)
•
V
IN
L
The resulting short-circuit current is:
I
SC
=
1/3 V
SENSE(MAX)
R
SENSE
–
1
2
∆I
L SC
( )
After a short, or while starting with internal soft-start, make
sure that the load current takes the folded-back current
limit into account.
Figure 12. Setup for a 5V Input
Figure 13. Setting Output Voltage
APPLICATIONS INFORMATION
R
VIN
1Ω
C
IN
3867 F12
5V
C
INTVCC
4.7µF
+
INTV
CC
LTC3867
V
IN
LTC3867
V
FB
V
OUT
R
B
C
FF
R
A
3867 F13
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LTC3867IUF#PBF

Mfr. #:
Buy LTC3867IUF#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Synchronous Step-Down DC/DC Controller with Differential Remote Sense and Non-Linear Control
Lifecycle:
New from this manufacturer.
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