ML4865
FUNCTIONAL DESCRIPTION
The ML4865 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, eliminate an external component, and
allows for load disconnect. Also included on-chip are an
NMOS switch and current sense resistor, further reducing
the number of external components, which makes the
ML4865 very easy to use.
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.
REGULATOR OPERATION
The ML4865 is a variable frequency, current mode
switching regulator. Its unique control scheme converts
efficiently over more than three decades of load current.
A detailed block diagram of the boost converter is shown
in Figure 2.
Error amplifier A3 converts deviations in the desired
output voltage to a small current, I
SET
. The inductor
current is measured through a 150mW 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 1.2A.
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
25µA of supply current is drawn from the output. This
allows the part to remain efficient even when the load
current drops below 250µA.
SHUTDOWN
The SHDN pin should be held low for normal operation.
Raising the shutdown voltage above the threshold level
will disable the synchronous rectifier, Q2 and Q3, and
force I
SET
to zero. This prevents switching from occurring
and disconnects the body diode of Q2 from the output. As
a result, the output voltage is allowed to drop below the
input voltage and current is prevented from flowing from
the input to the output.
FEEDBACK
The SENSE pin should be left open or bypassed to ground
for normal operation. The addition of the resistor divider
R1 and R2 causes the input of error amplifier A3 to reach
the threshold voltage before the internal resistors do. This
allows the ML4865 to provide output voltages lower than
the preset 12V if desired.
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
