ml4895 Microsemi Corporation, ml4895 Datasheet - Page 4

• Manufacturer Part Number: ml4895
• Description: Synchronous Buck Controller
• Manufacturer: Microsemi Corporation

ML4895
FUNCTIONAL DESCRIPTION
The ML4895 converts a 5.9V to 15V input to an
adjustable 2.5V to 4V output using a unique current
mode PFM synchronous buck control architecture. The
output current is set by external components, and can
exceed 2A. Even at light loads, the PFM architecture
maintains high conversion efficiencies over a wide range
of input voltages. If it is necessary to further extend
battery life, the user can shutdown and fully disconnect
the load from the input when the supply is not in use.
BIAS CIRCUITS
The bias circuits are comprised of a linear regulator and a
precision 2.5V reference. The V
bypassed to GND with a 1µF capacitor. The 2.5V
reference is used by the feedback circuit of the controller
to maintain an accurate output voltage.
SHUTDOWN LOGIC
The ML4895 is shut down by applying a logic low to the
SHDN pin. This prevents switching from occurring and
disconnects the load from the input. The supply current in
shutdown typically ranges from 0.5µA at V
3µA at V
BUCK CONTROLLER
A block diagram of the buck controller is shown in Figure
1. The circuit utilizes a constant ON-time PFM control
architecture. The circuit determines the OFF-time by
waiting for the inductor current to drop to a level set by
the feedback voltage (V
The oscillator/one shot block generates a constant ON-
time and a minimum OFF-time. The OFF-time is extended
for as long as the output of the current comparator stays
low. Note that the inductor current flows in the current
sense resistor during the OFF-time. Therefore, a minimum
OFF-time is required to allow for the finite circuit delays
in sensing the inductor current. The ON-time is triggered
when the current comparator’s output goes high.
However, unlike conventional fixed ON-time controllers,
this one shot has an inverse relationship with the input
voltage as shown in Figure 2. Figure 3 plots the inductor
voltage-ON-time product. Note that the volt-second
product is nearly constant over the entire input voltage
range. The inductor current is given by:
This means that the ripple current also remains nearly
constant over the entire input voltage range.
The transconductance amplifier generates a current from
the voltage difference between the reference and the
feedback voltage, V
across R
developed across the current sense resistor. When the
current level in the inductor drops low enough (a less
4
I
L
gm
IN
T
= 15V
ON
that adds to the negative voltage that is
b
V
IN
L
FB
. This current produces a voltage
V
FB
OUT
).
g
REG
pin should be
IN
= 5.9V to
(1)
negative sense voltage) to cause the voltage at the non-
inverting input of the current comparator to go positive,
the comparator trips and starts a new ON cycle. In other
words, the current programming comparator controls the
length of the OFF-time by waiting until the inductor
current decreases to a value determined by the
transconductance amplifier.
This technique allows the feedback transconductance
amplifier’s output current to steer the current level in the
inductor. The higher the transconductance amplifier’s
output current, the higher the inductor current. For
example, when the output voltage drops due to a load
increase, the transconductance amplifier will increase its
output current and generate a larger voltage across R
which in turn raises the inductor current trip level,
shortening the OFF-time. At some level of increasing the
output load, the transconductance amplifier can no
longer continue to increase its output current. When this
occurs, the voltage across R
the inductor current cannot increase. If the inductor
current tries to increase, the voltage developed across the
current sense resistor would become more negative,
causing the non-inverting input of the current comparator
to be negative, which extends the OFF-time and reduces
the inductor current.
If the output voltage is too high, the transconductance
amplifier’s output current will eventually become
negative. However, since the inductor current flows in
only one direction (assuming no shoot-through current)
the non-inverting input of the current comparator will
also stay negative. This extends the OFF-time allowing the
inductor current to decrease to zero, causing the
converter to stop operation until the output voltage drops
enough to increase the output current of the
transconductance amp above zero.
In summary, the three operation modes can be defined by
the voltage at the I
The synchronous rectifier comparator, flip-flop, and NOR
gate make up the synchronous rectifier control circuit.
The synchronous control does not influence the operation
of the main control loop, and operation with a Schottky
diode in place of the synchronous rectifier is possible, but
at a lower conversion efficiency. The synchronous rectifier
(N DRV) is turned on during the minimum OFF-time. N
DRV will remain on until a new ON-time is started or
until the I
goes above -7mV, the current in the inductor has gone to
zero or the buck regulator is operating in discontinuous
current mode (DCM). Therefore, the synchronous rectifier
comparator is used only for DCM operation. A timing
diagram is shown in Figure 4.
V
0V > V
-60mV > V
SENSE
SENSE
SENSE
> 0V - Discontinuous current mode
SENSE
> -60mV - Continuous current mode
pin goes above -7mV. When the I
SENSE
> -100mV - Current limit
pin at the end of the OFF-time:
gm
reaches a maximum and
SENSE
gm
pin
,