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LT3972EDD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3972
13
3972fa
Frequency Compensation
The LT3972 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular,
the LT3972 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V
C
pin, as shown in Figure 2. Generally a capacitor (C
C
)
and a resistor (R
C
) in series to ground are used. In addi-
tion, there may be lower value capacitor in parallel. This
capacitor (C
F
) is not part of the loop compensation but
is used to fi lter noise at the switching frequency, and is
required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
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 stabil-
ity using a transient load. Figure 2 shows an equivalent
circuit for the LT3972 control loop. The error amplifi er is a
transconductance amplifi er with fi nite output impedance.
The power section, consisting of the modulator, power
switch and inductor, is modeled as a transconductance
amplifi er 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 amplifi er 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 cur-
rent is stepped from 1A to 3A and back to 1A.
–
+
0.8V
SW
V
C
g
m
=
500μmho
GND
3M
LT3972
3972 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 5.3mho
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 3. Transient Load Response of the LT3972 Front Page
Application as the Load Current is Stepped from 1A to 3A.
V
OUT
= 5V
Figure 2. Model for Loop Response
3972 F03
I
L
1A/DIV
V
OUT
100mV/DIV
10μs/DIV
V
IN
= 12V
V
OUT
= 3.3V
APPLICATIONS INFORMATION
LT3972
14
3972fa
Low Ripple Burst Mode Operation
and Pulse-Skipping Mode
The LT3972 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 effi ciency at light loads, the LT3972 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 LT3972 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 LT3972 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
30μA and 90μA respectively during the sleep time. As
the load current decreases towards a no-load condition,
the percentage of time that the LT3972 operates in sleep
mode increases and the average input current is greatly
reduced resulting in high effi ciency even at very low loads.
See Figure 4. At higher output loads (above 140mA for
the front page application) the LT3972 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.
If low quiescent current is not required the LT3972 can
operate in pulse-skipping mode. The benefi t of this mode
is that the LT3972 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. Select pulse-skipping mode by applying
a clock signal or a DC voltage higher than 0.9V to the
SYNC pin.
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age 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 effi ciency. 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
another supply greater than 2.8V. Tying BD to V
IN
reduces
the maximum input voltage to 28V. The circuit in Figure 5a
is more effi cient 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.
The minimum operating voltage of an LT3972 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3972 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
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
Figure 4. Burst Mode Operation
3972 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
APPLICATIONS INFORMATION
LT3972
15
3972fa
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
GND
LT3972
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
D2
GND
LT3972
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
GND
LT3972
3972 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
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3972, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3972,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC fi lter to
create a voltage ramp at this pin. Figure 7 shows the start-
up and shutdown waveforms with the soft-start circuit.
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20μA when the RUN/SS
pin reaches 2.5V.
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 discontinu-
ous 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
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3972 F06
LOAD CURRENT (mA)
1
INPUT VOLTAGE (V)
4.0
4.5
5.0
10000
3.5
3.0
2.0
10 100 1000
1 1000010 100 1000
2.5
6.0
5.5
TO START
(WORST CASE)
TO RUN
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
5.0
6.0
7.0
4.0
2.0
3.0
8.0
TO RUN
V
OUT
= 3.3V
T
A
= 25°C
L = 8.2μH
f = 700kHz
V
OUT
= 5V
T
A
= 25°C
L = 8.2μH
f = 700kHz
TO START
(WORST CASE)
APPLICATIONS INFORMATION
Figure 5. Three Circuits For Generating The Boost Voltage
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LT3972EDD#PBF

Mfr. #:
Buy LT3972EDD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 36V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75uA Quiescent Current
Lifecycle:
New from this manufacturer.
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