LTC3401 datasheet(8/16 Pages) LINER

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LTC3401

3401fa

APPLICATIO S I FOR ATIO

Current Sensing. Lossless current sensing converts the

peak current signal to a voltage to sum in with the internal

slope compensation. This summed signal is compared to

the error amplifier output to provide a peak current control

command for the PWM. The slope compensation in the IC

is adaptive to the input and output voltage. Therefore, the

converter provides the proper amount of slope compensa-

tion to ensure stability and not an excess causing a loss of

phase margin in the converter.

Error Amp. The error amplifier is a transconductance

amplifier with gm = 0.1ms. A simple compensation net-

work is placed from the VC pin to ground.

Current Limit. The current limit amplifier will shut the

NMOS switch off once the current exceeds its threshold.

The current amplifier delay to output is typically 50ns.

Zero Current Amp. The zero current amplifier monitors the

inductor current to the output and shuts off the synchro-

nous rectifier once the current is below 50mA, preventing

negative inductor current.

Antiringing Control. The anitringing control will place an

impedance across the inductor to damp the ringing on the

SW pin during discontinuous mode operation. The LCSW

ringing (L = inductor, CSW = capacitance on the switch pin)

is low energy, but can cause EMI radiation.

Burst Mode Operation

Burst Mode operation is when the IC delivers energy to the

output until it is regulated and then goes into a sleep mode

where the outputs are off and the IC is consuming only

µA. In this mode, the output ripple has a variable

frequency component with load current and the steady

state ripple will be typically below 3%.

During the period where the device is delivering energy to

the output, the peak current will be equal to 1/3 the current

limit value and the inductor current will terminate at zero

current for each cycle. In this mode the maximum output

current is given by:

Amps

OUT MAXBURST

OUT

()

•

≈

DETAILED DESCRIPTION

The LTC3401 provides high efficiency, low noise power

for applications such as portable instrumentation. The

current mode architecture with adaptive slope compensa-

tion provides ease of loop compensation with excellent

transient load response. The low RDS(ON), low gate charge

synchronous switches provide the pulse width modula-

tion control at high efficiency.

The Schottky diode across the synchronous PMOS switch

provides a lower drop during the break-before-make time

(typically 20ns) of the NMOS to PMOS transition. The

Schottky diode improves efficiency (see graph “Efficiency

loss without Schottky vs Frequency”). While the IC’s

quiescent current is a low 38

µA,highefficiencyisachieved

at light loads when Burst Mode operation is entered.

Low Voltage Start-Up

The LTC3401 is designed to start up at input voltages of

typically 0.85V. The device can start up under some load,

(see graph “Start-Up vs Input Voltage”). Once the output

voltage exceeds a threshold of 2.3V, the IC 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 below 0.5V without affecting the opera-

tion, but the limiting factor for the application becomes the

availability of the power source to supply sufficient energy

to the output at the low voltages.

Low Noise Fixed Frequency Operation

Oscillator. The frequency of operation is set through a

resistor from the Rt pin to ground:

f = 3 • 1010/Rt

An internally trimmed timing capacitor resides inside the

IC. The oscillator can be synchronized with an external

clock inserted on the MODE/SYNC pin. When synchroniz-

ing the oscillator, the free running frequency must be set

to approximately 30% lower than the desired synchro-

nized frequency. Keeping the sync pulse width below 2

µs

will ensure that Burst Mode operation is disabled.

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