3433EFE データシートの表示（PDF） - Linear Technology

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3433EFE

High Voltage Step-Up/Step-Down DC/DC Converter

Linear Technology 

LT3433

APPLICATIO S I FOR ATIO

The requirement for avoiding current mode instability is

that the rising slope of sensed inductor ripple current (S1)

is greater than the falling slope (S2). At duty cycles greater

than 50% this is not true. To avoid the instability condition,

a false signal is added to the sensed current with a slope

(SX) that is sufficient to prevent current mode instability,

or S1 + SX ≥ S2. This leads to the following relations:

SX ≥ S2(2DC – 1)/DC

If the forward voltages of a converter’s catch and pass

diodes are defined as VF1 and VF2, then:

S2 = (VOUT + VF1 + VF2)/L

Solving for L yields a relation for the minimum inductance

that will satisfy slope compensation requirements:

LMIN = (VOUT + VF1 + VF2)(2DC – 1)/(DC • SX)

The LT3433 maximizes available dynamic range using a

slope compensation generator that generates a continu-

ously increasing slope as duty cycle increases. The slope

compensation waveform is calibrated at 80% duty cycle to

generate an equivalent slope of at least 0.05A/µs. The

equation for minimum inductance then reduces to:

LMIN = (VOUT + VF1 + VF2)(15e-6)

For example, with VOUT = 5V and using VF1 + VF2 = 1.1V

(cold):

LMIN = (5 + 1.1)(15e-6) = 91.5µH

Converter Capabilities

The output current capability of an LT3433 converter is

affected by a myriad of variables. The current in the

switches is limited by the LT3433. Switch current is

measured coming from the VIN supply, and does not

directly translate to a limitation in load current. This is

especially true during bridged mode operation when the

converter output current is discontinuous.

During bridged mode operation, the converter output

current is discontinuous, or only flowing to the output

while the switches are off (not to be confused with discon-

tinuous switcher operation). As a result, the maximum

output current capability of the converter is reduced from

that during buck mode operation by a factor of roughly

1 – DC, not including additional losses. Most converter

losses are also a function of DC, so operational duty cycle

must be accurately determined to predict converter load

capabilities.

VIN

SW_H

LT3433

SW_L

L

D2

D1

VOUT

3433 AI02

Slope Compensation Requirements

Typical Minimum Inductor Values vs VOUT

350

300

250

200

150

100

50

4 6 8 10 12 14 16 18 20

VOUT (V)

3433 AI01

10

Application variables:

VIN = Converter input supply voltage

VOUT = Converter programmed output voltage

VBST = Boosted supply voltage (VBST – VSWH)

DC = Operational duty cycle

fO = Switching frequency

IMAX = Peak switch current limit

∆IL = Inductor ripple current

ISW = Average switch current or peak switch current

less half the ripple current (IMAX – ∆IL/2)

RSWH = Boosted switch “on” resistance

RSWL = Grounded switch “on” resistance

L = Inductor value

3433f

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