ML4872
FUNCTIONAL DESCRIPTION
The ML4872 combines a unique form of current mode
control with a synchronous rectifier to create a boost
converter that can deliver high currents while maintaining
high efficiency. Current mode control allows the use of a
very small, high frequency inductor and output capacitor.
Synchronous rectification replaces the conventional
external Schottky diode with an on-chip PMOS FET to
reduce losses and eliminate an external component. Also
included on-chip are an NMOS switch and current sense
resistor, further reducing the number of external
components, which makes the ML4872 very easy to use.
DESIGN CONSIDERATIONS
OUTPUT CURRENT CAPABILITY
The maximum current available at the output of the
regulator is related to the maximum inductor current by
the ratio of the input to output voltage and the full load
efficiency. The maximum inductor current is
approximately 1.25A and the full load efficiency may be
as low as 70%. The maximum output current can be
determined by using the typical performance curves
shown in Figures 4 and 5, or by calculation using the
following equation:
REGULATOR OPERATION
The ML4872 is a variable frequency, current mode
switching regulator. Its unique control scheme converts
efficiently over more than three decades of load current.
A block diagram of the boost converter is shown in Figure
2.
Error amp A3 converts deviations in the desired output
voltage to a small current, I
SET
. The inductor current is
measured through a 50mW resistor which is amplified by
A1. The boost control block matches the average inductor
current to a multiple of the I
SET
current by switching Q1
on and off. The peak inductor current is limited by the
controller to about 1.5A.
At light loads, I
SET
will momentarily reach zero after an
inductor discharge cycle , causing Q1 to stop switching.
Depending on the load, this idle time can extend to
tenths of seconds. While the circuit is not switching, only
20µA of supply current is drawn from the output. This
allows the part to remain efficient even when the load
current drops below 200µA.
Amplifier A2 and the PMOS transistor Q2 work together
to form a low drop diode. When transistor Q1 turns off,
the current flowing in the inductor causes pin 6 to go
high. As the voltage on V
L2
rises above V
OUT
, amplifier
A2 allows the PMOS transistor Q2 to turn on. In
discontinuous operation, (where I
L
always returns to zero),
A2 uses the resistive drop across the PMOS switch Q2 to
sense zero inductor current and turns the PMOS switch
off. In continuous operation, the PMOS turn off is
independent of A2, and is determined by the boost control
circuitry.
Typical inductor current and voltage waveforms are
shown in Figure 3.
I
OUT( MAX)
=
125
.
V
V
IN( MIN)
OUT
0.7A
(1)
INDUCTOR SELECTION
The ML4872 is able to operate over a wide range of
inductor values. A value of 10µH is a good choice, but
any value between 5µH and 33µH is acceptable. As the
inductor value is changed the control circuitry will
automatically adjust to keep the inductor current under
control. Choosing an inductance value of less than 10µH
will reduce the component’s footprint, but the efficiency
and maximum output current may drop.
It is important to use an inductor that is rated to handle
1.5A peak currents without saturating. Also look for an
inductor with low winding resistance. A good rule of
thumb is to allow 5 to 10mW of resistance for each µH of
inductance.
The final selection of the inductor will be based on trade-
offs between size, cost and efficiency. Inductor tolerance,
core and copper loss will vary with the type of inductor
selected and should be evaluated with a ML4872 under
worst case conditions to determine its suitability.
Several manufacturers supply standard inductance values
in surface mount packages:
Coilcraft
Coiltronics
Dale
Sumida
(847) 639-6400
(561) 241-7876
(605) 665-9301
(847) 956-0666
SHUTDOWN
The SHDN pin should be held low for normal operation.
Raising the shutdown voltage above the threshold level
will disable the synchronous rectifier and force I
SET
to
zero. This prevents switching from occurring, and the
output voltage becomes V
IN
– V
DIODE
.
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
