lt3980 Linear Technology Corporation, lt3980 Datasheet - Page 13

• Manufacturer Part Number: lt3980
• Description: 58v, 2a, 2.4mhz Step-down Switching Regulator With 85?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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 LT3980 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
integrates this current, and that the capacitor on the V
(C
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
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
the feedback divider may improve the transient response.
Figure 3 shows the transient response when the load cur-
rent is stepped from 0.5A to 1.5A and back to 0.5A.
C
Figure 3. Transient Load Response of the LT3980 Front Page
Application as the Load Current Is Stepped from 0.5A to 1.5A
) integrates the error amplifi er output current, resulting
100mV/DIV
0.5A/DIV
V
C
OUT
I
in series with C
L
V
V
IN
OUT
= 12V
= 5V
C
pin. Note that the output capacitor
50μs/DIV
C
. This simple model works
3980 F03
PL
) across
C
pin
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 effi ciency 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
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 effi ciency 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
operating in standard PWM mode. The transition between
PWM and low ripple Burst Mode operation is seamless,
and will not disturb the output voltage.
IN
and BD quiescent currents are reduced to typically
10mV/DIV
0.2A/DIV
5V/DIV
V
V
OUT
SW
I
L
Figure 4. Burst Mode Operation
V
V
I
LOAD
IN
OUT
= 12V
= 3.3V
= 10mA
5μs/DIV
T
resistor, and will be
3980 F04
LT3980
13
3980f