BL8534
DETAILED DESCRIPTION
The BL8534 series are boost structure,
voltage-type Pulse-Frequency Modulation
(PFM) step-up DC-DC converter. Only three
external components are necessary: an
inductor, an output filter capacitor and a
schottky diode. And the converter’s low noise
and low ripple output voltage can be adjusted
from 2.5V to 5.0V, 0.1V step. By using the
depletion techniques, the quiescent current
drawn from power source is lower than 7uA.
The high efficiency device consists of
resistors for output voltage detection and
trimming, a start-up voltage circuit, an
oscillator, a reference circuit, a PFM control
circuit, a switch protection circuit and a driver
transistor.
The PFM control circuit is the core of the
BL8534. This block controls power switch on
duty cycle to stabilize output voltage by
calculating results of other blocks which
sense input voltage, output voltage, output
current and load conditions. In PFM
modulation system, the frequency and pulse
width is fixed. The duty cycle is adjusted by
skipping pulses, so that switch on-time is
changed based on the conditions such as
input voltage, output current and load. The
oscillate block inside BL8534 provides fixed
frequency and pulse width wave.
The reference circuit provides stable
reference voltage to output stable output
voltage.
Because
internal
trimming
technology is used, the chip output change
less than ±2%. At the same time
，
the
problem of temperature-drift coefficient of
output voltage is considered in design, so
temperature-drift coefficient of output voltage
is less than 100ppm/℃。
High-gain
differential
error
amplifiers
guarantees stable output voltage at difference
input voltage and load. In order to reduce
ripple and noise, the error amplifier is
designed with high band-with.
Though at very low load condition, the
quiescent current of chip do affect efficiency
certainly. The four main energy loss of Boost
structure DC-DC converter in full load are the
ESR of inductor, the voltage of Schottky
diode, on resistor of internal N-channel
MOSFET and its driver. In order to improve
the efficiency, BL8534 integrates low on-
resistor N-channel MOSFET and well design
driver circuits. The switch energy loss is
limited at very low level.
COMPONENTS SELECTION
Thus it can be seen, the inductor and
schottky diode affect the conversion
efficiency greatly. The inductor and the
capacitor also have great influence on the
output ripple voltage of the converter. So it is
necessary to choose a suitable inductor, a
capacitor and a right schottky diode, to obtain
high efficiency, low ripple and low noise.
Before discussion，we define
boost DC-DC to operate in the continuous
current-mode condition.
L
MIN
D(
1
−
D)
2
R
L
≥
2
f
D
≡
V
OUT
−
V
IN
V
OUT
INDUCTOR SELECTION
Above all, we should define the minimum
value of the inductor that can ensure the
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5
The above expression is got under conditions
of continuous current mode, neglect Schottky
diode’s voltage, ESR of both inductor and
capacitor. The actual value is greater that it. If
inductor’s value is less than L
min
，
the
efficiency of DC-DC converter will drop
greatly, and the DC-DC circuit will not be
stable.
Secondly, consider the ripple of the output
voltage,
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