BL8021 datasheet(5/7 Pages) BELLING

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BL8021

DETAILED DESCRIPTION

The BL8021 high-efficiency switching regulator is a

small, simple, DC-to-DC step-down converter

capable of delivering up to 1A of output current.

The device operates in pulse-width modulation

(PWM) at 1.5MHz from a 2.6V to 5.5V input voltage

and provides an output voltage from 0.6V to VIN,

making the BL8021 ideal for on-board post-

regulation applications. An internal synchronous

rectifier improves efficiency and eliminates the

typical Schottky free-wheeling diode. Using the on

resistance of the internal high-side MOSFET to

sense switching currents eliminates current-sense

resistors, further improving efficiency and cost.

Loop Operation

BL8021 uses a PWM current-mode control scheme.

An open-loop comparator compares the integrated

voltage-feedback signal against the sum of the

amplified current-sense signal and the slope

compensation ramp. At each rising edge of the

internal clock, the internal high-side MOSFET turns

on until the PWM comparator terminates the on

cycle. During this on-time, current ramps up

through the inductor, sourcing current to the

output and storing energy in the inductor. The

current mode feedback system regulates the peak

inductor current as a function of the output voltage

error signal. During the off cycle, the internal high-

side P-channel MOSFET turns off, and the internal

low-side N-channel MOSFET turns on. The inductor

releases the stored energy as its current ramps

down while still providing current to the output.

Current Sense

An internal current-sense amplifier senses the

current through the high-side MOSFET during on

time and produces a proportional current signal,

which is used to sum with the slope compensation

signal. The summed signal then is compared with

the error amplifier output by the PWM comparator

to terminate the on cycle.

Current Limit

There is a cycle-by-cycle current limit on the high-

side MOSFET of 1.5A(typ). When the current

flowing out of SW exceeds this limit, the high-side

MOSFET turns off and the synchronous rectifier

turns on. BL8021 utilizes a frequency fold-back

mode to prevent overheating during short-circuit

output conditions. The device enters frequency

fold-back mode when the FB voltage drops below

200mV, limiting the current to 1.5A (typ) and

reducing power dissipation. Normal operation

resumes

upon

removal

the

short-circuit

condition.

Soft-start

BL8021 has a internal soft-start circuitry to reduce

supply inrush current during startup conditions.

When the device exits under-voltage lockout

(UVLO), shutdown mode, or restarts following a

thermal-overload event, the l soft-start circuitry

slowly ramps up current available at SW.

UVLO and Thermal Shutdown

If IN drops below 2.5V, the UVLO circuit inhibits

switching. Once IN rises above 2.6V, the UVLO

clears, and the soft-start sequence activates.

Thermal-overload protection limits total power

dissipation in the device. When the junction

temperature exceeds TJ= +160°C, a thermal sensor

forces the device into shutdown, allowing the die to

cool. The thermal sensor turns the device on again

after the junction temperature cools by 15°C,

resulting in a pulsed output during continuous

overload conditions. Following a thermal-shutdown

condition, the soft-start sequence begins.

Design Procedure

Setting Output Voltages

Output voltages are set by external resistors. The

FB_ threshold is 0.6V.

RTOP = RBOTTOM[(VOUT / 0.6) - 1]

Input Capacitor Selection

The input capacitor in a DC-to-DC converter reduces

current peaks drawn from the battery or other

input power source and reduces switching noise in

the controller. The impedance of the input

capacitor at the switching frequency should be less

than that of the input source so high-frequency

BL8021 Datasheet(PDF) 5 Page - SHANGHAI BELLING CO., LTD.

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