LT1941 Просмотр технического описания (PDF) - Linear Technology

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LT1941

Triple Monolithic Switching Regulator

Linear Technology 

LT1941

APPLICATIONS INFORMATION

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 10μF. 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

magnitudes 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 example

a 1μF ceramic capacitor in parallel with a low ESR tantalum

capacitor. For the electrolytic capacitor, a value larger than

10μF 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, tan-

talum capacitors should be surge rated. The manufacturer

may also recommend operation below the rated voltage

of the capacitor. Be sure to place the 1μF 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 plugging

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 capacitor

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 application

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 voltage

and temperature. The LT1375 data sheet contains a more

thorough discussion of loop compensation and describes

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 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 cur-

rent 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 ampli-

fier 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 (ΔIRIPPLE > 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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