BL8542B
BLOCK DIAGRAM
OUT
EN
osc
Reference
Low Input startup
+
GM
+
Comparator
FB
-
Logic
Driver
SW
-
+
-
G
DETAIL DESCRIPTION
BL8542B is a low input voltage start up,
current mode dc-dc step up converter.
It’s operation can be best understood
by referring to the block diagram. Upon
star ting up, the low voltage star tup
circuitry drives SW with on-off cycles,
transferring energy from input to OUT
by storing energy in the inductor during
on-time and releasing it to the output
during off-time. When OUT reaches
2V, the startup circuit turns off and the
main controller takes over. The main
control loop consists of a reference, a
GM error amplifier, a PWM controller, a
current sense amplifier, an oscillator, a
PWM logic control, and it is power stage
including its driver. The main control
loop is a classic current mode control
loop. The GM stage integrates the error
between FB and REF, and its output is
used to compare with a triangular wave
which the summing result of the current
sense amplifier output and a slope
compensation voltage. The output of the
comparator is used to drive the power
stage to reach regulation.
APPLICATION INFORMATION
Output Voltage selection
The output voltages can be set by connecting FB to OUT, to G or to
the midpoint of a resistor divider connected to OUT. See below table
for details.
FB=G
FB=OUT
FB to resistor divider
VOUT =5.0V
VOUT =3.3V
VOUT=0.6V(1+R1/R2)
Inductor selection
With switching frequency up to 1MHz, small surface mount inductors
can be used with values from 2.2uH to 4.7uH. For a given chosen
inductor value and application conditions make sure the peak inductor
current does not exceed the maximum current rating of the selected
vendor's inductor.
Input and output capacitor selection
The BL8542B's bootstrap architecture allows the use of very small
input capacitor. For applications that only need to drive small output
load current, the input capacitor is optional, because once output is
started up, the IC's is powered by OUT, a quiet power supply.
The output capacitor is used to stabilize the loop and provide ac
current to the load. A low ESR ceramic cap with values from 2.2uF
to 22uF can be used. Smaller value capacitors are generally cheaper
with small footprints, while larger capacitor provides lower ripples
and better transient load responses. Also, when extreme low startup
voltage is needed, larger output capacitors are needed for the part to
startup under heavy load condition.
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