LTC3422 Просмотр технического описания (PDF) - Linear Technology

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LTC3422

1.5A, 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect

Linear Technology 

LTC3422

U

OPERATIO

inrush current limiting are provided during start-up as

well as normal switching mode. The same soft-start

capacitor is used for each operating mode.

When either VIN or VOUT exceeds 2.25V, the LTC3422

enters normal operating mode. Once the output voltage

exceeds the input by 0.3V typical, the LTC3422 powers

itself from VOUT instead of VIN. At this point the internal

circuitry has no dependency on the VIN input voltage,

eliminating the requirement for a large input capacitor.

The input voltage can drop as low as 0.5V without affecting

circuit operation. The limiting factor for the application

becomes the availability of the power source to supply

sufficient energy to the output at the low voltages and the

maximum duty cycle, which is clamped at 91% typical.

LOW NOISE FIXED FREQUENCY OPERATION

Shutdown

The part is shutdown by pulling SHDN below 0.25V, and

activated by pulling the pin initially above 1V. Once VOUT

exceeds 2.2V typical, hysteresis is applied to this pin

allowing it to maintain a logic high state down to 0.65V.

Note that SHDN can be driven above VIN or VOUT as long

as it is limited to less than the absolute maximum rating.

Soft-Start

The soft-start time is programmed with an external capaci-

tor from SS to ground. An internal current source charges

it with a nominal 2.4µA. The ramping voltage on SS

dictates the gradually increasing peak current limit until

the voltage on the capacitor exceeds 1.6V, after which the

internally set peak current limit is maintained. In the event

of a commanded shutdown or a thermal shutdown, the

capacitor on SS is discharged to ground automatically.

Note that Burst Mode operation is inhibited during the

soft-start time.

t (ms) = CSS (µF) • 320

Oscillator

The frequency of operation is set through a resistor from

RT to ground. A precision timing capacitor resides inside

the LTC3422. The oscillator can be synchronized with an

external clock applied to SYNC. When synchronizing the

8

oscillator, the free running frequency must be set at least

20% lower than the desired synchronized frequency.

fOSC

=

28

RT

where fOSC is in MHz and RT is in kΩ.

Current Sensing

Lossless current sensing converts the peak current signal

to a voltage to sum in with the internal slope compensa-

tion. This summed signal is compared to the error ampli-

fier output to provide a peak current control command for

the PWM. The LTC3422 incorporates slope compensation

which is adaptive to the input and output voltages. There-

fore, the converter provides the proper amount of slope

compensation to ensure stability, but not an excess which

would cause a loss of phase margin in the converter.

Error Amplifier

The error amplifier is a transconductance amplifier, with

its positive input internally connected to the 1.216V refer-

ence and its negative input connected to FB. A simple

compensation network is placed from VC to ground.

Internal clamps limit the minimum and maximum error

amplifier output voltage for improved large-signal tran-

sient response.

Current Limit

The current limit circuitry shuts off the internal N-channel

MOSFET switch when the current limit threshold is reached.

In Burst Mode operation, the current limit is reduced to

approximately 600mA.

Zero Current Amplifier

The zero current amplifier monitors the inductor current to

the output and shuts off the synchronous rectifier once the

current falls below 50mA typical, preventing negative

inductor current.

Anti-Ringing Control

The anti-ringing control connects a resistor across the

inductor to dampen the ringing on SW during discontinu-

ous conduction mode. The LCSW ringing (L = inductor,

3422fa

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