5

ML4771

FUNCTIONAL DESCRIPTION

The ML4771 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 ML4771 very easy to use.

REGULATOR OPERATION

The ML4771 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

measured through a 50m

W resistor which is amplified by

A1. The boost control block matches the average inductor

on and off. The peak inductor current is limited by the

controller to about 1.5A.

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 PMOS transistor Q2 to turn on. In discontinuous

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.

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:

I

V

V

A

OUT MAX

IN MIN

OUT

()

()

..





125

0 7

(1)

INDUCTOR SELECTION

The ML4771 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 10m

W 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 ML4771 under

worst case conditions to determine its suitability.

Several manufacturers supply standard inductance values

in surface mount packages:

Coilcraft

(847) 639-6400

Coiltronics

(561) 241-7876

Dale

(605) 665-9301

Sumida

(847) 956-0666