BL8021
1.2A 1.5MHz 7V Synchronous Buck Converter
DESCRIPTION
The BL8021 is a high-efficiency, DC-to-DC step-
down switching regulators, capable of delivering up
to 1.2A of output current. The device operates from
an input voltage range of 2.6V to 7.0V and provides
an output voltage from 0.6V to VIN, making the
BL8021 ideal for low voltage power conversions.
Running at a fixed frequency of 1.5MHz allows the
use of small external components, such as ceramic
input and output caps, as well as small inductors,
while still providing low output ripples. This low
noise output along with its excellent efficiency
achieved by the internal synchronous rectifier,
making BL8021 an ideal green replacement for
large power consuming linear regulators. Internal
soft-start control circuitry reduces inrush current.
Short-circuit and thermal-overload protection
improves design reliability.
BL8021 is housed in a SOT23-5 Package
FEATURES
High Efficiency: Up to 96%
Capable of Delivering 1.2A
1.5MHz Switching Frequency
No External Schottky Diode Needed
Low dropout 100% Duty operation
Internal Compensation and Soft-Start
Current Mode control
0.6V Reference for Low Output voltages
Logic Control Shutdown (IQ<1uA)
Thermal shutdown and UVLO
Available in SOT23-5
APPLICATIONS
Cellular phones
Digital Cameras
MP3 and MP4 players
Set top boxes
Wireless and DSL Modems
USB supplied Devices in Notebooks
Portable Devices
TYPICAL APPLICATION
V
IN
2.6V to 7V
IN
SW
2.2uH
PIN OUT & MARKING
V
OUT
1.8V/1A
EN 1
5
FB
10uF
EN
LC3406
BL8021
FB
GND
240K
10uF
GGZZ
G 2
SW 3
120K
4 IN
SOT23-5
1.8V/1A Step-Down Converter
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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.
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
condition.
removal
of
the
short-circuit
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.
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.
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.
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.
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
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
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