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LTC3857IGN-1#PBF

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LTC3857-1
19
38571fc
APPLICATIONS INFORMATION
Figure 5. Setting Output Voltage
RMS capacitor current requirement. Increasing the out-
put current drawn from the other controller will actually
decrease the input RMS ripple current from its maximum
value. The out-of-phase technique typically reduces the
input capacitor’s RMS ripple current by a factor of 30%
to 70% when compared to a single phase power supply
solution.
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
OUT
)/(V
IN
). To prevent
large voltage transients, a low ESR capacitor sized for the
maximum RMS current of one channel must be used. The
maximum RMS capacitor current is given by:
C
IN
Required I
RMS
≈
I
MAX
V
IN
V
OUT
()
V
IN
–V
OUT
()
⎡
⎣
⎤
⎦
1/ 2
(1)
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3857-1, ceramic capacitors
can also be used for C
IN
. Always consult the manufacturer
if there is any question.
The benefit of the LTC3857-1 2-phase operation can be
calculated by using Equation 1 for the higher power control-
ler and then calculating the loss that would have resulted
if both controller channels switched on at the same time.
The total RMS power lost is lower when both controllers
are operating due to the reduced overlap of current pulses
required through the input capacitor’s ESR. This is why
the input capacitor’s requirement calculated above for the
worst-case controller is adequate for the dual controller
design. Also, the input protection fuse resistance, battery
resistance, and PC board trace resistance losses are also
reduced due to the reduced peak currents in a 2-phase
system. The overall benefit of a multiphase design will
only be fully realized when the source impedance of the
power supply/battery is included in the efficiency testing.
The drains of the top MOSFETs should be placed within
1cm of each other and share a common C
IN
(s). Separating
the drains and C
IN
may produce undesirable voltage and
current resonances at V
IN
.
A small (0.1µF to 1µF) bypass capacitor between the chip
V
IN
pin and ground, placed close to the LTC3857-1, is
also suggested. A 10 resistor placed between C
IN
(C1)
and the V
IN
pin provides further isolation between the
two channels.
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (∆V
OUT
) is approximated by:
ΔV
OUT
≈ΔI
L
ESR +
1
8•f•C
OUT
⎛
⎝
⎜
⎞
⎠
⎟
where f is the operating frequency, C
OUT
is the output
capacitance and ∆I
L
is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since ∆I
L
increases with input voltage.
Setting Output Voltage
The LTC3857-1 output voltages are each set by an exter-
nal feedback resistor divider carefully placed across the
output, as shown in Figure 5. The regulated output voltage
is determined by:
V
OUT
= 0.8V 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.
1/2 LTC3857-1
V
FB
V
OUT
R
B
C
FF
R
A
38571 F05
LTC3857-1
20
38571fc
APPLICATIONS INFORMATION
(7a) Coincident Tracking
(7b) Ratiometric Tracking
Figure 8. Using the TRACK/SS Pin for Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of each V
OUT
is controlled by the voltage on
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference, the
LTC3857-1 regulates the V
FB
pin voltage to the voltage on
the TRACK/SS pin instead of 0.8V. The TRACK/SS pin can
be used to program an external soft-start function or to
allow V
OUT
to track another supply during start-up.
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 6.
An internal 1µA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3857-1 will regulate the V
FB
pin (and hence V
OUT
)
according to the voltage on the TRACK/SS pin, allowing
V
OUT
to rise smoothly from 0V to its final regulated value.
The total soft-start time will be approximately:
t
SS
= C
SS
•
0.8V
1µ A
TIME
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
38571 F07a
TIME
38571 F07b
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
1/2 LTC3857-1
V
OUT
V
x
V
FB
TRACK/SS
38571 F08
R
B
R
A
R
TRACKA
R
TRACKB
1/2 LTC3857-1
TRACK/SS
C
SS
SGND
38571 F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies during start-up, as shown qualitatively
in Figures 7a and 7b. To do this, a resistor divider should
be connected from the master supply (V
X
) to the TRACK/
SS pin of the slave supply (V
OUT
), as shown in Figure 8.
During start-up V
OUT
will track V
X
according to the ratio
set by the resistor divider:
V
X
V
OUT
=
R
A
R
TRACKA
•
R
TRACKA
+ R
TRACKB
R
A
+ R
B
For coincident tracking (V
OUT
= V
X
during start-up):
R
A
= R
TRACKA
R
B
= R
TRACKB
LTC3857-1
21
38571fc
APPLICATIONS INFORMATION
INTV
CC
Regulators
The LTC3857-1 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the INTV
CC
pin from either the V
IN
supply pin or the
EXTV
CC
pin depending on the connection of the EXTV
CC
pin. INTV
CC
powers the gate drivers and much of the
LTC3857-1’s internal circuitry. The V
IN
LDO and the EXTV
CC
LDO regulate INTV
CC
to 5.1V. Each of these can supply a
peak current of 50mA and must be bypassed to ground
with a minimum of 4.7µF ceramic capacitor. No matter
what type of bulk capacitor is used, an additional 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 the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3857-1 to
be exceeded. The INTV
CC
current, which is dominated
by the gate charge current, may be supplied by either
the V
IN
LDO or the EXTV
CC
LDO. When the voltage on
the EXTV
CC
pin is less than 4.7V, the V
IN
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 3 of the
Electrical Characteristics. For example, the LTC3857-1
INTV
CC
current is limited to less than 15mA from a 40V
supply when not using the EXTV
CC
supply at a 70°C
ambient temperature:
T
J
= 70°C + (15mA)(40V)(90°C/W) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in forced continuous mode (PLLIN/MODE
= INTV
CC
) at maximum V
IN
.
When the voltage applied to EXTV
CC
rises above 4.7V, the
V
IN
LDO is turned off and the EXTV
CC
LDO is enabled. The
EXTV
CC
LDO remains on as long as the voltage applied to
EXTV
CC
remains above 4.5V. The EXTV
CC
LDO attempts
to regulate the INTV
CC
voltage to 5.1V, so while EXTV
CC
is less than 5.1V, the LDO is in dropout and the INTV
CC
voltage is approximately equal to EXTV
CC
. When EXTV
CC
is greater than 5.1V, up to an absolute maximum of 14V,
INTV
CC
is regulated to 5.1V.
Using the EXTV
CC
LDO allows the MOSFET driver and
control power to be derived from one of the LTC3857-1’s
switching regulator outputs (4.7V ≤ V
OUT
≤ 14V) during
normal operation and from the V
IN
LDO when the out-
put is out of regulation (e.g., start-up, short-circuit). If
more current is required through the EXTV
CC
LDO than
is specified, an external Schottky diode can be added
between the EXTV
CC
and INTV
CC
pins. In this case, 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 the output, since the V
IN
cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 14V regulator outputs, this means connecting
the EXTV
CC
pin directly to V
OUT
. Tying the EXTV
CC
pin to
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T
J
= 70°C + (15mA)(8.5V)(90°C/W) = 82°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 5.1V regulator result-
ing 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 to 14V regulator and provides the
highest efficiency.
3.
EXTV
CC
Connected to an External supply. If an external
supply is available in the 5V to 14V range, it may be
used to power EXTV
CC
. Ensure that EXTV
CC
< V
IN
.
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LTC3857IGN-1#PBF

Mfr. #:
Buy LTC3857IGN-1#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Low IQ, Dual, 2-Phase Synchronous Step Down Controller
Lifecycle:
New from this manufacturer.
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