ML4790
Note, that at lower output voltages there is less voltage
required at the PFM stage, and therefore less gate drive
available for the pass device Q3. This results in Q3 being
more resistive and unable to deliver as much output
current as a ML4790 set for a higher output voltage. This
characteristic is shown in Figure 4.
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.
200
180
160
140
120
100
80
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:
60
40
20
0
2.5V
3.5V
4.5V
5.5V
V
2 × TON(MIN) × η
V
(V)
IN(MIN)
OUT
LMAX
=
(1)
2× (VOUT + VOS) ×IOUT(MAX)
Figure 4. ML4790 I
V
MAX
OUT
where η is the efficiency, typically between 0.75 and
= V
– 0.5V, L = 22µH
IN
OUT
0.85, and V is the dropout voltage at I taken
OS
OUT(MAX)
PFM REGULATOR OPERATION
from Figure 3. 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.
When the output of the PFM stage, V
(pin 5), is at or
BOOST
OS
above the dropout voltage, V
+ V , the output of A1
OUT
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
Another method of determining the appropriate inductor
value is to make an estimate based on the typical
performance curves given in Figures 6 and 7. Figure 6
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 5.5V application requires
40mA of output current while using an inductor with 15%
tolerance. The output current should be derated by 25%
to 50mA to cover the combined inductor and ON-time
tolerances. Assuming that 2V is the end of life voltage of a
two cell input, Figure 6 shows that with a 2V input, the
ML4790 delivers 52mA with a 22µH inductor.
inductor current in Q2 falls to zero, the output of A2 goes
high, releasing Q2‘s gate, allowing the flip-flop to be SET
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 5.
INDUCTOR
CURRENT
Q2
ON
Q2
ON
Q(ONE SHOT)
Q1 ON
Q1 ON
Q1 & Q2 OFF
Figure 5. PFM Inductor Current
Waveforms and Timing.
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
