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LT3980IDE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3980
7
3980fa
For more information www.linear.com/LT3980
SYNC (Pin 1/Pin 1): This is the external clock synchro-
nization input. Ground this pin for low ripple Burst Mode
operation at low output loads. Tie to a voltage above 0.8V
to select pulse-skipping mode. Tie to a clock source for
synchronization. Clock edges should have rise and fall
times faster than 1µs. Tie pin to GND if not used. See the
Synchronization section in Applications Information.
NC (Pins 2, 13/Pins 3, 5, 13, 16): No Connect. These
pins are not connected to internal circuitry.
PG (Pin 3/Pin 2): The PG pin is the open collector output
of an internal comparator. PG remains low until the FB pin
is within 9% of the final regulation voltage. PG output is
valid when V
IN
is above 3.6V and RUN/SS is high.
FB (Pin 4/Pin 4): The LT3980 regulates the FB pin to 0.790V.
Connect the feedback resistor divider tap to this pin.
V
C
(Pin 5/Pin 6): The V
C
pin is the output of the internal
error amplifier. The voltage on this pin controls the peak
switch current. Tie an RC network from this pin to ground
to compensate the control loop.
RT (Pin 6/Pin 7): Oscillator Resistor Input. Connecting
a resistor to ground from this pin sets the switching
frequency.
GND (Pin 7, 15/Pin 8, 17): Ground. The exposed pads
must be soldered to the PCB.
RUN/SS (Pin 8/Pin 9): The RUN/SS pin is used to put the
LT3980 in shutdown mode. Tie to ground to shut down
the LT3980. Tie to 2.5V or more for normal operation. If
the shutdown feature is not used, tie this pin to the V
IN
pin. RUN/SS also provides a soft-start function; see the
Applications Information section.
BD (Pin 9/Pin 10): This pin connects to the anode of the
boost Schottky diode. BD also supplies current to the
internal regulator.
BOOST (Pin 10/Pin 11): This pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pin 11/Pin 12): The SW pin is the output of the
internal power switch. Connect this pin to the inductor,
catch diode and boost capacitor.
V
IN
(Pin 12/Pin 14): The V
IN
pin supplies current to the
LT3980’s internal regulator and to the internal power switch.
This pin must be locally bypassed.
DA (Pin 14/Pin 15): This pin measures catch diode current
and pauses the oscillator during overcurrent conditions.
+
–
+
–
+
–
OSCILLATOR
100kHzTO2.4MHz
BurstMode
DETECT
V
C
CLAMP
SOFT-START
SLOPE COMP
R
V
IN
V
IN
RUN/SS
BOOST
SW
SWITCH
LATCH
V
C
V
OUT
C2
C3
C
F
L1
D1
DISABLE
C
C
R
C
BD
RT
R2
GND
ERROR AMP
R1
FB
R
T
C1
PG
0.725V
S
Q
Σ
DA
3680 BD
INTERNAL 0.79V REF
SYNC
block DiaGram
pin FuncTions
(DFN, MSOP)
LT3980
8
3980fa
For more information www.linear.com/LT3980
The LT3980 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
enables an RS flip-flop, turning on the internal power
switch. An amplifier and comparator monitor the current
flowing between the V
IN
and SW pins, turning the switch
off when this current reaches a level determined by the
voltage at V
C
. An error amplifier measures the output
voltage through an external resistor divider tied to the FB
pin and servos the V
C
pin. If the error amplifier’s output
increases, more current is delivered to the output; if it
decreases, less current is delivered. An active clamp on the
V
C
pin provides current limit. The V
C
pin is also clamped to
the voltage on the RUN/SS pin; soft-start is implemented
by generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the V
IN
pin,
but if the BD pin is connected to an external voltage higher
than 3V bias power will be drawn from the external source
(typically the regulated output voltage). This improves
efficiency. The RUN/SS pin is used to place the LT3980
in shutdown, disconnecting the output and reducing the
input current to less than 0.5µA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate the
internal bipolar NPN power switch for efficient operation.
To further optimize efficiency, the LT3980 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input supply
current to 75µA in a typical application.
The oscillator reduces the LT3980’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during startup
and overload. In addition, the LT3980 monitors the catch
diode current flowing through the DA pin and pauses the
oscillator during overcurrent conditions to keep inductor
current at safe levels.
The LT3980 contains a power good comparator which trips
when the FB pin is at 91% of its regulated value. The PG
output is an open-collector transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3980 is
enabled and V
IN
is above 3.6V.
The LT3980 has an overvoltage protection feature which
disables switching action when the V
IN
goes above 61.5V
typical (58V minimum). When switching is disabled, the
LT3980 can safely sustain input voltages up to 62V.
operaTion
LT3980
9
3980fa
For more information www.linear.com/LT3980
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis
-
tors according to:
RR
V
V
OUT
12
079
1=
.
–
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3980 uses a constant frequency PWM architecture
that can be programmed to switch from 100kHz to 2.4MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary RT value for a desired switching
frequency is in Figure 1.
SWITCHING FREQUENCY (MHz) R
T
VALUE (kΩ)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
1.8
2.0
432
215
137
97.6
76.8
60.4
51.1
43.2
35.7
32.4
24.9
20
16.2
14
11
Figure 1. Switching Frequency vs R
T
Value
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values may
be used. The disadvantages are lower efficiency
, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (f
SW(MAX)
) for a
given application can be calculated as follows:
f
VV
tVVV
SW MAX
D OUT
ON MIN
DINSW
()
()
=
+
+
()
–
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
D
is the catch diode drop (~0.5V) and V
SW
is the
internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V
IN
/V
OUT
ratio. Also, as shown
in the next section, lower frequency allows a lower dropout
voltage. The reason input voltage range depends on the
switching frequency is because the LT3980 switch has
finite minimum on and off times. The switch can turn on
for a minimum of ~200ns and turn off for a minimum of
~200ns. This means that the minimum and maximum
duty cycles are:
DC ft
DC ft
MINSW
ON MIN
MAXSW
OFFMIN
=
=
()
()
1–
where f
SW
is the switching frequency, the t
ON(MIN)
is the
minimum switch on time (~200ns), and the t
OFF(MIN)
is
the minimum switch off time (~200ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
A good choice of switching frequency should allow ade
-
quate input voltage range (see next section) and keep the
inductor and capacitor values small.
Input V
oltage Range
The maximum input voltage for LT3980 applications de-
pends on switching frequency, Absolute Maximum Ratings
of the V
IN
and BOOST pins, and the operating mode.
The LT3980 can operate from input voltages of up to 58V,
and withstand voltages up to 80V. Note that while V
IN
is
above 61V typical (58V minimum and 64V maximum)
the part will keep the switch off and the output will not
be in regulation.
T
he switching frequency should be chosen according to
the following equation:
V
VV
ft
VV
IN MAX
OUT D
SW
ON MIN
DS
W
()
()
=
+
+–
where V
IN(MAX)
is the maximum typical operating input
voltage, V
OUT
is the output voltage, V
D
is the catch diode
applicaTions inFormaTion
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LT3980IDE#PBF

Mfr. #:
Buy LT3980IDE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 58V, 2A, 2.4MHz Step-Down Switching Regulator with 75uA Quiescent Current
Lifecycle:
New from this manufacturer.
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