ML4895

4

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

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

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 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:

∆I

TV

V

L

L

ON

IN

OUT

=

×−

bg

(1)

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

developed across the current sense resistor. When the

current level in the inductor drops low enough (a less

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

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

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 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

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.