LP6221A datasheet(6/7 Pages) POWER

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LP6221A

Feb.-2019

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Preliminary Datasheet

LP6221A

Application Information

The LP6221A uses a fixed frequency, peak current

mode boost regulator architecture to regulate voltage

at the feedback pin. At the start of each oscillator

cycle the MOSFET is turned on through the control

circuitry. To prevent sub-harmonic oscillations at

duty cycles greater than 50 percent, a stabilizing

ramp is added to the output of the current sense

amplifier and the result is fed into the negative input

of the PWM comparator. When this voltage equals

the output voltage of the error amplifier the power

MOSFET is turned off. The voltage at the output of

the error amplifier is an amplified version of the

difference between the 1.155V bandgap reference

voltage and the feedback voltage. In this way the

peak current level keeps the output in regulation. If

the feedback voltage starts to drop, the output of the

error amplifier increases. These results in more

current to flow through the power MOSFET, thus

increasing the power delivered to the output. The

LP6221A has internal soft start to limit the amount of

input current at startup and to also limit the amount of

overshoot on the output.

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. The voltage divider drops the

output voltage to the 1.155V feedback voltage. Use a

20K resistor for RDN of the voltage divider. Determine

the high-side resistor RUP by the equation:

VOUT = (

RUP

RDN

+ 1) × VFB

Current Limitation

The internal power-MOS switch current is monitored

cycle-by-cycle and is limited to the value not exceed

10A(Typ.). When the switch current reaches the

limited value, the internal power-MOS is turned off

immediately until the next cycle. Keep traces at this

pin as short as possible. Do not put capacitance at

this pin.

Current limit program

A resistor between OC and GND pin programs peak

switch current. When the ROC value is between 10K

and 30K, the current limit will be set from 10A to 2A.

If the OC pin is left floating, the default current limit is

6A. Keep traces at this pin as short as possible. Do

not put capacitance at this pin.

Diode Selection

To achieve high efficiency, Schottky diode is good

choice for low forward drop voltage and fast

switching time. The output diode rating should be

able to handle the maximum output voltage, average

power dissipation and the pulsating diode peak

current.

Output Disconnect Control Function

The LP6221A can control the external MOS switch

through the OD pin. When EN is low, the OD pin is in

a high-impedance state. When EN is high, the OD

pin is pulled down to GND. This allows the

asynchronous boost to cut off the output in shutdown

mode.

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