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LT1941

1941f

APPLICATIO S I FOR ATIO

and is largest when VIN = 2 VOUT (50% duty cycle). As the

second, lower power channel draws input current, the

input capacitor’s RMS current actually decreases as the

out-of-phase current cancels the current drawn by the

higher power channel. The ripple current contribution

from the third channel will be minimal. Considering that

the maximum load current from a single channel is ~2.8A,

RMS ripple current will always be less than 1.4A.

The high frequency of the LT1941 reduces the energy

storage requirements of the input capacitor, so that the

capacitance required is often less than 10

µF. The combi-

nation of small size and low impedance (low equivalent

series resistance or ESR) of ceramic capacitors makes

them the preferred choice. The low ESR results in very low

voltage ripple. Ceramic capacitors can handle larger mag-

nitudes of ripple current than other capacitor types of the

same value. Use X5R and X7R types.

An alternative to a high value ceramic capacitor is a lower

value along with a larger electrolytic capacitor, for ex-

ample a 1

µF ceramic capacitor in parallel with a low ESR

tantalum capacitor. For the electrolytic capacitor, a value

larger than 10

µF will be required to meet the ESR and

ripple current requirements. Because the input capacitor

is likely to see high surge currents when the input source

is applied, tantalum capacitors should be surge rated. The

manufacturer may also recommend operation below the

rated voltage of the capacitor. Be sure to place the 1

µF

ceramic as close as possible to the VIN and GND pins on

the IC for optimal noise immunity.

A final caution is in order regarding the use of ceramic

capacitors at the input. A ceramic input capacitor can

combine with stray inductance to form a resonant tank

circuit. If power is applied quickly (for example by plug-

ging the circuit into a live power source), this tank can ring,

doubling the input voltage and damaging the LT1941. The

solution is to either clamp the input voltage or dampen the

tank circuit by adding a lossy capacitor in parallel with the

ceramic capacitor. For details, see Application Note 88.

Frequency Compensation

The LT1941 uses current mode control to regulate the

output. This simplifies loop compensation. In particular, the

LT1941 does not depend on the ESR of the output capaci-

tor for stability so you are free to use ceramic capacitors

to achieve low output ripple and small circuit size.

The components tied to the VC pin provide frequency

compensation. Generally, a capacitor and a resistor in

series to ground determine loop gain. In addition, there is

a lower value capacitor in parallel. This capacitor filters

noise at the switching frequency and is not part of the loop

compensation.

Loop compensation determines the stability and transient

performance. Designing the compensation network is a

bit complicated and the best values depend on the appli-

cation and 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 compensation

network to optimize the performance. Check stability across

all operating conditions, including load current, input volt-

age and temperature. The LT1375 data sheet contains a

more thorough discussion of loop compensation and de-

scribes how to test the stability using a transient load.

Application Note 76 is an excellent source as well.

Figure 5 shows an equivalent circuit for the LT1941

control loop. The error amp is a transconductance ampli-

fier 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) integrates the error

amplifier output current, resulting in two poles in the loop.

In most cases, a zero is required and comes either from the

output capacitor ESR or from a resistor in series with CC.

This model works well as long as the inductor current

ripple is not too low (

∆IRIPPLE > 5% IOUT ) and the loop

crossover frequency is less than

ƒSW/5. A phase lead

capacitor (CPL) across the feedback divider may improve

the transient response.

The equivalent circuit for the LT1941 inverter control loop

is slightly different than is shown in Figure 5. The feedback

resistors are connected as shown for negative outputs in

Figure 2. The operational amplifier is fast enough to have

minimal effect on the loop dynamics.

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