ML4890
Also included in the LDO stage is an offset voltage
FUNCTIONAL DESCRIPTION
control. This circuit monitors the output current and
adjusts the offset voltage according the general
characteristic shown in Figure 3. The offset control
ensures that the PFM stage provides just enough
“overhead” voltage for the LDO stage to operate properly.
The ML4890 combines Pulse Frequency Modulation
(PFM) and synchronous rectification to create a boost
converter that is followed by a low dropout linear
regulator (LDO). This combination creates a low output
ripple boost converter that is both highly efficient and
simple to use.
PFM REGULATOR OPERATION
When the output of the PFM stage, V
(pin 5), is at or
BOOST
The 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.
above the dropout voltage, V
+ V , the output of A1
OUT
OS
stays low and the circuit remains idle. When V
falls
BOOST
below the required dropout voltage, the output of A1 goes
high, signaling the regulator to deliver charge to the
capacitor C2. Since the output of A2 is normally high, the
output of the flip-flop becomes SET. This triggers the one
shot to turn Q1 on and begins charging L1 for 5µs. When
the one shot times out, Q1 turns off, allowing L1 to
flyback and momentarily charge C2 through the body
diode of Q2. But, as the source voltage of Q2 rises above
the drain, the current sensing amplifier A2 drives the gate
of Q2 low, causing Q2 to short out the body diode. The
inductor then discharges into C2 through Q2. The output
of A2 going low also serves to RESET the flip-flop in
preparation for the next charging cycle. When the
The integrated LDO reduces the output ripple voltage to
less than 5mV peak-to-peak. Integrating the LDO along
with the PFM regulator allows the circuit to be optimized
for very high efficiency using a patented feedback
technique. It also allows the LDO to provide the
maximum ripple rejection over the operating frequency
range of the regulator.
inductor current in Q2 falls to zero, the output of A2 goes
high, releasing Q2‘s gate, allowing the flip-flop to be SET
A block diagram of the ML4890 is shown in Figure 2. The
PFM stage is comprised of Q1, Q2, A1, A2, the one shot,
the flip-flop, and externals L1 and C2. The LDO stage is
comprised of Q3, A3, R1, R2, the offset voltage control,
and external C1. Since the LDO actually controls the
operation of the PFM regulator, the operation of the LDO
stage will be covered first.
again. If the voltage at V
is still low, A1 will initiate
BOOST
another pulse. Typical inductor current and voltage
waveforms are shown in Figure 4.
INDUCTOR
CURRENT
LDO OPERATION
The LDO stage operates as a linear regulator. A3 is the
error amplifier, which compares the output voltage
through the divider R1 and R2 to the reference, and Q3 is
the pass device. When the output voltage is lower than
desired, the output of A3 increases the gate drive of Q3,
which reduces the voltage drop across it and brings the
output back into regulation. Similarly, if the output voltage
is higher than desired, A3 adjusts the gate drive of Q3 for
more drop and the output is brought back into regulation.
Q2
Q2
ON
Q(ONE SHOT)
Q1 ON
Q1 ON
ON
Q1 & Q2 OFF
Figure 4. PFM Inductor Current Waveforms and Timing.
SHUTDOWN
The SHDN pin should be held low for normal operation.
Raising the voltage on SHDN above the threshold level
will release the gate of Q3, which effectively becomes an
open circuit. This also prevents the one shot from
triggering, which keeps switching from occurring.
450
400
350
300
250
200
150
100
0
10 20 30 40 50 60 70 80 90 100
(mA)
I
OUT
Figure 3. LDO V versus output current.
OS
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
