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LT3433

3433ia

The LT3433 contains circuitry to eliminate the current limit

reduction associated with slope-compensation, or anti-

slope compensation. As the slope compensation ramp is

added to the sensed current, a similar ramp is added to the

current limit threshold reference. The end result is that

current limit is not compromised so the LT3433 can

provide full power regardless of required duty cycle.

Mode Switching

The LT3433 senses operational duty cycle by directly

monitoring VIN and VOUT. Voltage drops associated with

pass and catch diodes are estimated internally such that

mode switching occurs when the duty cycle required for

continuous buck operation is greater than 75%. If such a

condition exists, a second switch is enabled during the

switch on time, changing operation to a dual switch

bridged configuration. Because the voltage available across

the switched inductor is greater in bridged mode, duty

cycle will decrease.

The output current in bridged mode is not continuous, so

switch currents are considerably higher than while oper-

ating in buck mode. In order to maximize available output

power, continuous operation and low ripple currents are

recommended. Switch currents will increase by a factor of

1/(1 – DC) during bridged mode, so this mode of operation

is typically the gating item for converter drive capability.

IOUT(MAX) = ISW(MAX) • (1 – DC)

= [0.5A – (

∆IL / 2)] • (1 – DC)

where

∆IL is the ripple current in the inductor.

It is also important to note that IOUT cannot be considered

equivalent to ILOAD during bridged operation. Most of the

converter’s switch drive power is derived from the gener-

ated output supply, so IOUT must also accommodate this

current requirement. During single-switch buck opera-

tional conditions, switch drive current is negligible in

terms of output current; however, during bridged opera-

tion, these currents can become significant. These output

derived switch drive currents will increase the current

loading on VIN by the same 1/(1 – DC) factor as the switch

currents. As maximum switch current is referenced to that

coming from the VIN supply, the available maximum

switch current will be reduced by this required drive

current.

IDRIVE = DC • 2 • ISW(MAX) • ISWDRIVE(MAX)

Using 50mA/A for the required drive current for each

switch yields the portion of switch current used to drive

the switches is:

ISW(DRIVE) = DC • 2 • ISW(MAX) • 0.05/(1 – DC)

Removing drive currents from the available maximum

switch current yields:

ISW(MAX)'= ISW(MAX) • [1 – DC • 2 • ISW(MAX) •

0.05/(1 – DC)]

where ISW(MAX)' is maximum switch current available to

the load during bridged operation. The maximum load

current can then be calculated as:

ILOAD(MAX) = ISW(MAX)' • (1 – DC)

which reduces to:

ILOAD(MAX) = [0.5A – (∆IL/2)] • (1 – 1.1 • DC)

Design Equations

APPLICATIO S I FOR ATIO

SW_H

LT3433

VIN

VOUT

3433 AI01

SW_L

Constants:

VSWH = voltage drop across boosted switch

VSWL = voltage drop across grounded switch

VF = forward drop of external Schottky diodes

f0 = operating frequency

Duty Cycle (continuous operation):

DCBUCK = (VOUT + 2VF)/(VIN – VSWH + VF)

DCBRIDGED = (VOUT + 2VF)/(VOUT + VIN + 2VF – VSWH

– VSWL)

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