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LT1940

Dual Monolithic 1.4A, 1.1MHz Step-Down Switching Regulator

Linear Technology 

LT1940/LT1940L

APPLICATIO S I FOR ATIO

The boost circuit can also run directly from a DC voltage

that is higher than the input voltage by more than 3V, as in

Figure 3d. The diode is used to prevent damage to the

LT1940 in case VIN2 is held low while VIN is present. The

circuit saves several components (both BOOST pins can

be tied to D2). However, efficiency may be lower and

dissipation in the LT1940 may be higher. Also, if VIN2 is

absent, the LT1940 will still attempt to regulate the output,

but will do so with very low efficiency and high dissipation

because the switch will not be able to saturate, dropping

1.5V to 2V in conduction.

The minimum input voltage of an LT1940 application is

limited by the minimum operating voltage (< 3.6V) and by

the maximum duty cycle as outlined above. For proper

start-up, the minimum input voltage is also limited by the

boost circuit. If the input voltage is ramped slowly, or the

LT1940 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. The Typical Performance Characteris-

tics section shows plots of the minimum load current to

start and to run as a function of input voltage for 3.3V and

5V outputs. In many cases the discharged output capaci-

tor will present a load to the switcher which will allow it to

start. The plots show the worst-case situation where VIN

is ramping very slowly. Use a Schottky diode (such as the

BAT-54) for the lowest start-up voltage.

Frequency Compensation

The LT1940 uses current mode control to regulate the

output. This simplifies loop compensation. In particular,

the LT1940 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 VC pin. 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 is not part

12

of the loop compensation but is used to filter noise at the

switching frequency.

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 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

compensation network to optimize the performance. Sta-

bility should then be checked across all operating condi-

tions, including load current, input voltage and tempera-

ture. The LT1375 data sheet contains a more thorough

discussion of loop compensation and describes how to

test the stability using a transient load.

Figure 4 shows an equivalent circuit for the LT1940

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 from either the

output capacitor ESR or from a resistor 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.

LT1940

CURRENT MODE

VSW

POWER STAGE

gm = 2.5mho

ERROR

AMPLIFIER

R1

gm =

340µmho

500k

FB

1.25V

GND

VC

CPL

ESR

+

C1

RC

CF

CC

POLYMER

R2

OR

TANTALUM

OUTPUT

C1

CERAMIC

Figure 4. Model for Loop Response

1940 F05

1940fa

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