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LT3980

3980fa

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Loop compensation determines the stability and transient

performance.Designingthecompensationnetworkisabit

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

circuit 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 VC pin. Note that the output capacitor

integrates this current, and that the capacitor on the VC pin

(CC)integratestheerroramplifieroutputcurrent,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 RC in series with CC. 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 (CPL) 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

theoutputwithsingle,lowcurrentpulses,theoutputripple

is kept below 15mV for a typical application. In addition,

VIN 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 RT 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

0.5A/DIV

VOUT

100mV/DIV

50µs/DIV

VIN = 12V

VOUT = 5V

applicaTions inFormaTion

Figure 4. Burst Mode Operation

3980 F04

0.2A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

5µs/DIV

VIN = 12V

VOUT = 3.3V

ILOAD = 10mA

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