LT3431 Просмотр технического описания (PDF) - Linear Technology

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LT3431

High Voltage, 3A, 500kHz Step-Down Switching Regulator

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LT3431

APPLICATIO S I FOR ATIO

A detailed theoretical basis for estimating internal power

dissipation is given in the Thermal Calculations section.

This will allow a first pass check of whether an application’s

maximum input voltage requirement is suitable for the

LT3431. Be aware that these calculations are for DC input

voltages and that input voltage transients as high as 60V

are possible if the resulting increase in internal power

dissipation is of insufficient time duration to raise die

temperature significantly. For the FE package, this means

high voltage transients on the order of hundreds of milli-

seconds are possible. If LT3431 thermal calculations

show power dissipation is not suitable for the given

application, the LT3430 is a recommended alternative

since it is identical to the LT3431 but runs cooler at

200kHz.

Switch minimum on time is the other factor that may limit

the maximum operational input voltage for the LT3431 if

pulse-skipping behavior is not allowed. For the LT3431,

pulse-skipping may occur for VIN/(VOUT + VF) ratios > 4.

(VF = Schottky diode D1 forward voltage drop, Figure 5.)

If the LT3430 is used, the ratio increases to 10. Pulse-

skipping is the regulator’s way of missing switch pulses to

maintain output voltage regulation. Although an increase

in output ripple voltage can occur during pulse-skipping,

a ceramic output capacitor can be used to keep ripple

voltage to a minimum (see output ripple voltage compari-

son for tantalum vs ceramic output capacitors, Figure 3).

FREQUENCY COMPENSATION

Before starting on the theoretical analysis of frequency

response, the following should be remembered—the worse

the board layout, the more difficult the circuit will be to

stabilize. This is true of almost all high frequency analog

circuits, read the Layout Considerations section first.

Common layout errors that appear as stability problems

are distant placement of input decoupling capacitor and/

or catch diode, and connecting the VC compensation to a

ground track carrying significant switch current. In addi-

tion, the theoretical analysis considers only first order

non-ideal component behavior. For these reasons, it is

important that a final stability check is made with produc-

tion layout and components.

The LT3431 uses current mode control. This alleviates

many of the phase shift problems associated with the

inductor. The basic regulator loop is shown in Figure 10.

The LT3431 can be considered as two gm blocks, the error

amplifier and the power stage.

LT3431

CURRENT MODE

POWER STAGE

gm = 2mho

SW

ERROR

AMPLIFIER

gm =

2000µmho

RO

200k

FB

1.22V

GND

VC

RC

CF

CC

CFB R1

RLOAD

R2

OUTPUT

CERAMIC

ESR

ESL

+

C1

C1

TANTALUM

3431 F10

Figure 10. Model for Loop Response

Figure 11 shows the overall loop response. At the VC pin,

the frequency compensation components used are:

RC = 3.3k, CC = 0.022µF and CF = 220pF. The output

capacitor used is a 100µF, 10V tantalum capacitor with

typical ESR of 100mΩ.

The ESR of the tantalum output capacitor provides a

useful zero in the loop frequency response for maintain-

ing stability.

This ESR, however, contributes significantly to the ripple

voltage at the output (see Output Ripple Voltage in the

Applications Information section). It is possible to reduce

capacitor size and output ripple voltage by replacing the

sn3431 3431fs

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