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LT3980EMSE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3980
13
3980fa
For more information www.linear.com/LT3980
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com
-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature. The
LT1375 data sheet contains a more thorough discussion
of loop compensation and describes how to test the sta
-
bility using a transient load. Figure 2 shows an equivalent
cir
cuit for the LT3980 control loop. The error amplifier is a
transconductance amplifier with finite output impedance.
The power section, consisting of the modulator, power
switch and inductor, is modeled as a transconductance
amplifier generating an output current proportional to
the voltage at the V
C
pin. Note that the output capacitor
integrates this current, and that the capacitor on the V
C
pin
(C
C
) integrates the error amplifier output current, resulting
in two poles in the loop. In most cases a zero is required
and comes from either the output capacitor ESR or from
a resistor R
C
in series with C
C
. This simple model works
well as long as the value of the inductor is not too high
and the loop crossover frequency is much lower than the
switching frequency. A phase lead capacitor (C
PL
) across
the feedback divider may improve the transient response.
Figure 3 shows the transient response when the load
current is stepped from 0.5A to 1.5A and back to 0.5A.
Low Ripple Burst Mode Operation and Pulse-Skipping
Mode
The LT3980 is capable of operating in either low ripple
Burst Mode operation or pulse-skipping mode which are
selected using the SYNC pin. See the Synchronization
section for details.
To enhance efficiency at light loads, the LT3980 can be
operated in low ripple Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst
Mode operation, the LT3980 delivers single cycle bursts of
current to the output capacitor followed by sleep periods
where the output power is delivered to the load by the
output capacitor. Because the LT3980 delivers power to
the output with single, low current pulses, the output ripple
is kept below 15mV for a typical application. In addition,
V
IN
and BD quiescent currents are reduced to typically
35µA and 82µA respectively during the sleep time. As
the load current decreases towards a no-load condition,
the percentage of time that the LT3980 operates in sleep
mode increases and the average input current is greatly
reduced resulting in high efficiency even at very low loads.
See Figure 4. At higher output loads (above 140mA for
the front page application) the LT3980 will be running at
the frequency programmed by the R
T
resistor, and will be
operating in standard PWM mode. The transition between
PWM and low ripple Burst Mode operation is seamless,
and will not disturb the output voltage.
Figure 3. Transient Load Response of the LT3980 Front Page
Application as the Load Current Is Stepped from 0.5A to 1.5A
3980 F03
I
L
0.5A/DIV
V
OUT
100mV/DIV
50µs/DIV
V
IN
= 12V
V
OUT
= 5V
applicaTions inFormaTion
Figure 4. Burst Mode Operation
3980 F04
I
L
0.2A/DIV
V
SW
5V/DIV
V
OUT
10mV/DIV
5µs/DIV
V
IN
= 12V
V
OUT
= 3.3V
I
LOAD
= 10mA
LT3980
14
3980fa
For more information www.linear.com/LT3980
If low quiescent current is not required the LT3980 can
operate in pulse-skipping mode. The benefit of this mode
is that the LT3980 will enter full frequency standard PWM
operation at a lower output load current than when in
Burst Mode operation. The front page application circuit
will switch at full frequency at output loads higher than
about 60mA.
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost voltage
that is higher than the input voltage. In most cases a
0.22µF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3V, use a 1µF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1µF boost
capacitor (see Figure 5b). For lower output voltages the
boost diode can be tied to the input (Figure 5c), or to an
-
other supply greater than 2.8V. Tying BD to V
IN
reduces
the maximum input voltage to 28V. The circuit in Figure 5a
is more efficient because the BOOST pin current and BD
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
applicaTions inFormaTion
Figure 5. Three Circuits for Generating the Boost Voltage
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT3980
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
D2
GND
LT3980
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT3980
3980 FO5
(5a) For V
OUT
> 2.8V
(5b) For 2.5V < V
OUT
< 2.8V
(5c) For V
OUT
< 2.5V; V
IN(MAX)
= 30V
LT3980
15
3980fa
For more information www.linear.com/LT3980
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 6 shows a plot
of minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
start. The plots show the worst-case situation where V
IN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V
IN
; however, this restricts the
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinuous
and the effective duty cycle can be very high. This reduces
the minimum input voltage to approximately 300mV above
V
OUT
. At higher load currents, the inductor current is
continuous and the duty cycle is limited by the maximum
duty cycle of the LT3980, requiring a higher input voltage
to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3980,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
While operating with high boost voltages (>10V), it is
important to ensure that the power dissipation from the
boost circuit is not too high. See the Typical Performance
Characteristics section for the plot, BOOST Pin Current
vs Switch Current. Boost circuit power dissipation is
calculated as follows:
P
BOOST
= I
BOOST
V
BOOST
– SW
DC
Where DC is the switch duty cycle, I
BOOST
is the boost pin
current, and V
BOOST
– V
SW
is the voltage between the boost
pin and switch pin. If the P
BOOST
> 0.5W, a Zener can be put
between the boost pin and the boost capacitor such that
the power is dissipated in the Zener instead of the LT3980.
The minimum operating voltage of an LT3980 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3980 is turned on with its RUN/SS pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
applicaTions inFormaTion
1 10 100 1000 2000
f = 400kHz
I
LOAD
(mA)
INPUT VOLTAGE (V)
4
5
3980 F06a
3
2
6
TO START
TO RUN
I
LOAD
(mA)
INPUT VOLTAGE (V)
6
7
3980 F06b
5
4
8
1 10 100 1000 2000
TO START
TO RUN
f = 400kHz
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LT3980EMSE#TRPBF

Mfr. #:
Buy LT3980EMSE#TRPBF
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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