ML4775CS Просмотр технического описания (PDF) - Micro Linear Corporation
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ML4775CS Datasheet PDF : 8 Pages
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FUNCTIONAL DESCRIPTION
The ML4775 combines Pulse Frequency Modulation
(PFM) and synchronous rectification to create a boost
converter that is both highly efficient and simple to use.
A PFM regulator charges a single inductor for a fixed
period of time and then completely discharges before
another cycle begins, simplifying the design by
eliminating the need for conventional current limiting
circuitry. Synchronous rectification is accomplished by
replacing an external Schottky diode with an on-chip
PMOS device, reducing switching losses and external
component count.
REGULATOR OPERATION
A block diagram of the boost converter is shown in
Figure 2. The circuit remains idle when VOUT is at or
above the desired output voltage, drawing 55µA from
VIN, and 8µA from VOUT through the feedback resistors
R1 and R2. When VOUT drops below the desired output
level, the output of amplifier A1 goes high, signaling the
regulator to deliver charge to the output. Since the
output of amplifier A2 is normally high, the flip-flop
captures the A1 set signal and creates a pulse at the gate
of the NMOS transistor Q1. The NMOS transistor will
charge the inductor L1 for 10µs, resulting in a peak
current given by:
IL(PEAK)
=
TON × VIN
L1
≈
10µs × VIN
L1
(1)
For reliable operation, L1 should be chosen so that
IL(PEAK) does not exceed 1.5A.
When the one-shot times out, the NMOS transistor
releases the VL pin, allowing the inductor to fly-back and
momentarily charge the output through the body diode
of PMOS transistor Q2 in series with shutdown transistor
Q3. But, as the voltage across the PMOS transistor
changes polarity, its gate will be driven low by the
current sense amplifier A2, causing Q2 to short out its
body diode. The inductor then discharges into the load
through Q2. The output of A2 also serves to reset the
flip-flop and one-shot in preparation for the next
charging cycle. A2 releases the gate of Q2 when its
current falls to zero. If VOUT is still low, the flip-flop will
immediately initiate another pulse. The output capacitor
(C1) filters the inductor current, limiting output voltage
ripple. Inductor current and one-shot waveforms are
shown in Figure 3.
INDUCTOR
CURRENT
Q(ONE SHOT)
Q2
Q1 ON
ON
Q1 & Q2 OFF
Q2
Q1 ON ON
Figure 3. PFM Inductor Current Waveforms and Timing.
ML4775
SHUTDOWN
The ML4775 output can be shut down by pulling the
SHDN pin high. When SHDN is high, the regulator stops
switching, the control circuitry is powered down, and
the body diode of the PMOS synchronous rectifier is
disconnected from the output, allowing the output
voltage to drop below the input voltage. This feature is
unique to the ML4775, as most boost regulators use
external Schottky diode rectifier which cannot be
disconnected during shutdown. Leaving the Schottky
diode connected causes excess power dissipation in the
load during shutdown because the Schottky conducts
whenever the output voltage drops 300mV below the
input voltage.
DESIGN CONSIDERATIONS
INDUCTOR
Selecting the proper inductor for a specific application
usually involves a trade-off between efficiency and
maximum output current. Choosing too high a value will
keep the regulator from delivering the required output
current under worst case conditions. Choosing too low a
value causes efficiency to suffer. It is necessary to know
the maximum required output current and the input
voltage range to select the proper inductor value. The
maximum inductor value can be estimated using the
following formula:
LMAX
=
VIN
2
(MIN)
×
TON
(MIN)
×
2 × VOUT × IOUT(MAX)
η
(2)
where h is the efficiency, typically between 0.8 and 0.9.
Note that this is the value of inductance that just barely
delivers the required output current under worst case
conditions. A lower value may be required to cover
inductor tolerance, the effect of lower peak inductor
currents caused by resistive losses, and minimum dead
time between pulses.
Another method of determining the appropriate inductor
value is to make an estimate based on the typical
performance curves given in Figures 4 and 5. Figure 4
shows maximum output current as a function of input
voltage for several inductor values. These are typical
performance curves and leave no margin for inductance
and ON-time variations. To accommodate worst case
conditions, it is necessary to derate these curves by at
least 10% in addition to inductor tolerance.
For example, a two cell to 5V application requires 80mA
of output current while using an inductor with 15%
tolerance. The output current should be derated by 25%
to 100mA to cover the combined inductor and ON-time
tolerances. Assuming that 2V is the end of life voltage of
a two cell input, Figure 4 shows that with a 2V input, the
ML4775 delivers 100mA with a 27µH inductor.
5
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