ML4871
FUNCTIONAL DESCRIPTION
The ML4871 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 ML4871 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 ML4871 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.
RESET
COMPARATOR
An additional comparator is provided to detect low V
IN
,
low V
OUT
, or any other error condition that the user may
want to sense. The inverting input of the comparator is
connected to the 1.25V reference, and the non-inverting
input is connected to the DETECT pin. The output of this
comparator is connected to the
RESET
pin of the device
and can swing from V
OUT
to ground.
I
OUT( MAX)
=
125
.
V
V
IN( MIN)
OUT
0.7A
(1)
INDUCTOR SELECTION
The ML4871 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 ML4871 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
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
