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LTC3865EUH-1#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36P37-P38
LTC3865/LTC3865-1
19
3865fb
APPLICATIONS INFORMATION
which are highest at high input voltages. For V
IN
< 20V
the high current effi ciency generally improves with larger
MOSFETs, while for V
IN
> 20V the transition losses rapidly
increase to the point that the use of a higher R
DS(ON)
device
with lower C
MILLER
actually provides higher effi ciency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diodes conduct during the dead time
between the conduction of the two power MOSFETs. These
prevent the body diodes of the bottom MOSFETs from turn-
ing on, storing charge during the dead time and requiring
a reverse recovery period that could cost as much as 3%
in effi ciency at high V
IN
. A 1A to 3A Schottky is generally
a good compromise for both regions of operation due to
the relatively small average current. Larger diodes result
in additional transition losses due to their larger junction
capacitance.
Soft-Start and Tracking
The LTC3865/LTC3865-1 have the ability to either soft-start
by themselves with a capacitor or track the output of another
channel or external supply. When one particular channel
is confi gured to soft-start by itself, a capacitor should be
connected to its TK/SS pin. This channel is in the shutdown
state if its RUN pin voltage is below 1.22V. Its TK/SS pin
is actively pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.22V, the channel pow-
ers up. A soft-start current of 1.3µA then starts to charge
its soft-start capacitor. Note that soft-start or tracking is
achieved not by limiting the maximum output current of
the controller but by controlling the output ramp voltage
according to the ramp rate on the TK/SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
defi ned to be the voltage range from 0V to 0.6V on the
TK/SS pin. The total soft-start time can be calculated as:
t
C
A
SOFTSTART
SS
= 06
13
.•
. μ
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode
up to TK/SS = 0.5V. Between TK/SS = 0.5V and 0.54V, it
will operate in forced continuous mode and revert to the
selected mode once TK/SS > 0.54V. The output ripple
is minimized during the 40mV forced continuous mode
window ensuring a clean PGOOD signal.
When the channel is confi gured to track another supply,
the feedback voltage of the other supply is duplicated by
a resistor divider and applied to the TK/SS pin. Therefore,
the voltage ramp rate on this pin is determined by the
ramp rate of the other supply’s voltage. Note that the small
soft-start capacitor charging current is always fl owing,
producing a small offset error. To minimize this error, select
the tracking resistive divider value to be small enough to
make this error negligible.
In order to track down another channel or supply after
the soft-start phase expires, the LTC3865/LTC3865-1 are
forced into continuous mode of operation as soon as V
FB
is below the undervoltage threshold of 0.54V regardless of
the setting on the MODE/PLLIN pin. However, the LTC3865/
LTC3865-1 should always be set in forced continuous
mode tracking down when there is no load. After TK/SS
drops below 0.1V, the corresponding channel will operate
in discontinuous mode.
Output Voltage Tracking
The LTC3865/LTC3865-1 allow the user to program how its
output ramps up and down by means of the TK/SS pins.
Through these pins, the output can be set up to either co-
incidentally or ratiometrically track another supply’s output,
as shown in Figure 5. In the following discussions, V
OUT1
refers to the LTC3865/LTC3865-1’s output 1 as a master
channel and V
OUT2
refers to the LTC3865/LTC3865-1’s out-
put 2 as a slave channel. In practice, though, either phase
can be used as the master. To implement the coincident
tracking in Figure 5a, connect an additional resistive divider
to V
OUT1
and connect its midpoint to the TK/SS pin of the
slave channel. The ratio of this divider should be the same
as that of the slave channel’s internal feedback divider
LTC3865/LTC3865-1
20
3865fb
APPLICATIONS INFORMATION
shown in Figure 6a. In this tracking mode, V
OUT1
must
be set higher than V
OUT2
. To implement the ratiometric
tracking, the ratio of the V
OUT2
divider should be exactly
the same as the master channel’s internal feedback divider.
By selecting different resistors, the LTC3865/LTC3865-1
can achieve different modes of tracking including the two
in Figure 5.
So which mode should be programmed? The coincident
mode offers better output regulation. This can be better
understood with the help of Figure 7. At the input stage
of the slave channel’s error amplifi er, two common an-
ode diodes are used to clamp the equivalent reference
voltage and an additional diode is used to match the shifted
common mode voltage. The top two current sources are
of the same magnitude. In coincident mode, the TK/SS
voltage is substantially higher than 0.6V at steady state and
effectively turns off D1. D2 and D3 will therefore conduct
the same current and offer tight matching between V
FB2
and the internal precision 0.6V at steady state. In the
ratiometric mode, however, TK/SS equals 0.6V at steady
state. D1 will divert part of the bias current to make V
FB2
slightly lower than 0.6V. Although this error is minimized
by the exponential I-V characteristics of the diode, it does
impose a fi nite amount of output voltage deviation.
TIME
(5a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
V
OUT1
V
OUT2
TIME
3865 F05
(5b) Ratiometric Tracking
OUTPUT VOLTAGE
Figure 5. Two Different Modes of Output Voltage Tracking
nR3 R1
nR4 R2
R3
V
OUT2
R4
(6a) Coincident Tracking Setup
TO
EA1
TO
TK/SS2
PIN
TO
EA2
V
OUT1
38551 F06
(6b) Ratiometric Tracking Setup
nR1 R1
nR2 R2
R3
V
OUT2
R4
TO
EA1
TO
TK/SS2
PIN
TO
EA2
V
OUT1
Figure 6. Setup for Coincident and Ratiometric Tracking
–
+
D3
3865 F07
D2D1
TK/SS2
0.6V
V
FB2
II
EA2
Figure 7. Equivalent Input Circuit of Error Amplifi er
LTC3865/LTC3865-1
21
3865fb
APPLICATIONS INFORMATION
When the master channel’s output experiences dynamic
excursion (under load transient, for example), the slave
channel output will be affected as well. For better output
regulation, use the coincident tracking mode instead of
ratiometric.
INTV
CC
Regulators and EXTV
CC
The LTC3865 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 LTC3865/LTC3865-1’s internal
circuitry. The linear regulator regulates the voltage at the
INTV
CC
pin to 5V when V
IN
is greater than 5.5V. 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. Each of these can supply a peak current of 80mA
and must be bypassed to ground with a minimum of 4.7µF
ceramic capacitor or low ESR electrolytic capacitor. No mat-
ter 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 supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
between channels.
High input voltage applications in which large MOSFETs are
being driven at high frequencies may cause the maximum
junction temperature rating for the LTC3865/LTC3865-1
to be exceeded. The INTV
CC
current, which is dominated
by the gate charge current, may be supplied by either the
5V linear regulator or EXTV
CC
. When the voltage on the
EXTV
CC
pin is less than 4.7V, the linear regulator 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 depen-
dent on operating frequency as discussed in the Effi ciency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 3 of the
Electrical Characteristics. For example, the LTC3865 INTV
CC
current is limited to less than 42mA from a 38V supply in
the UH package and not using the EXTV
CC
supply:
T
J
= 70°C + (42mA)(38V)(34°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/PLLIN
= SGND) at maximum V
IN
. When the voltage applied to
EXTV
CC
rises above 4.7V, the INTV
CC
linear regulator 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 one
of the LTC3865/LTC3865-1’s switching regulator outputs
during normal operation and from the INTV
CC
when the
output is out of regulation (e.g., start-up, short-circuit). If
more current is required through the EXTV
CC
than is speci-
fi ed, 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
.
Signifi cant effi ciency and thermal gains can be realized by
powering INTV
CC
from the output, since the V
IN
current
resulting from the driver and control currents will be scaled
by a factor of (Duty Cycle)/(Switcher Effi ciency).
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 + (42mA)(5V)(34°C/W) = 77°C
However, for 3.3V and other low voltage outputs, addi-
tional 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 5V regulator result-
ing in an effi ciency 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
effi ciency.
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, effi ciency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.7V.
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LTC3865EUH-1#TRPBF

Mfr. #:
Buy LTC3865EUH-1#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs
Lifecycle:
New from this manufacturer.
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