LTC3400/LTC3400B

5

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OPERATIO

The LTC3400/LTC3400B are 1.2MHz, synchronous boost

converters housed in a 6-lead ThinSOT package. Able to

operate from an input voltage below 1V, the devices

feature fixed frequency, current mode PWM control for

and gate charge internal MOSFET switches, the devices

maintain high efficiency over a wide range of load current.

Detailed descriptions of the three distinct operating modes

follow. Operation can be best understood by referring to

the Block Diagram.

Low Voltage Start-Up

age of 0.85V or higher. The low voltage start-up circuitry

controls the internal NMOS switch up to a maximum peak

inductor current of 850mA (typ), with an approximate

1.5

µs off-time during start-up, allowing the devices to

start-up circuitry is disabled and normal fixed frequency

PWM operation is initiated. In this mode, the LTC3400/

operating time as the battery can droop to several tenths

of a volt without affecting output voltage regulation. The

limiting factor for the application becomes the ability of the

battery to supply sufficient energy to the output.

Low Noise Fixed Frequency Operation

Oscillator: The frequency of operation is internally set to

1.2MHz.

Error Amp: The error amplifier is an internally compensated

transconductance type (current output) with a transconduc-

voltage is compared to the voltage at the FB pin to generate

an error signal at the output of the error amplifier. A volt-

voltage via FB from 2.5V to 5V using the equation:

Current Sensing: A signal representing NMOS switch

current is summed with the slope compensator. The

summed signal is compared to the error amplifier output

to provide a peak current control command for the PWM.

Peak switch current is limited to approximately 850mA

independent of input or output voltage. The current signal

is blanked for 40ns to enhance noise rejection.

Zero Current Comparator: The zero current comparator

monitors the inductor current to the output and shuts off

the synchronous rectifier once this current reduces to ap-

proximately 20mA. This prevents the inductor current from

reversing in polarity improving efficiency at light loads.

Antiringing Control: The antiringing control circuitry pre-

vents high frequency ringing of the SW pin as the inductor

current goes to zero by damping the resonant circuit

Burst Mode Operation

Portable devices frequently spend extended time in low

power or standby mode, only switching to high power

drain when specific functions are enabled. In order to

improve battery life in these types of products, high power

converter efficiency needs to be maintained over a wide

output power range. In addition to its high efficiency at

moderate and heavy loads, the LTC3400 includes auto-

matic Burst Mode operation that improves efficiency of

the power converter at light loads. Burst mode operation

is initiated if the output load current falls below an

internally programmed threshold (see Typical Perfor-

Once initiated, the Burst Mode operation circuitry shuts

down most of the device, only keeping alive the circuitry

required to monitor the output voltage. This is referred to

as the sleep state. In sleep, the LTC3400 draws only 19

µA

from the output capacitor, greatly enhancing efficiency.

When the output voltage has drooped approximately 1%

from nominal, the LTC3400 wakes up and commences

normal PWM operation. The output capacitor recharges

and causes the LTC3400 to reenter sleep if the output load

remains less than the sleep threshold. The frequency of

this intermittent PWM or burst operation is proportional to

load current; that is, as the load current drops further

below the burst threshold, the LTC3400 turns on less

frequently. When the load current increases above the