5

ML4851

July 2000

DATASHEET

FUNCTIONAL DESCRIPTION

The ML4851 combines Pulse Frequency Modulation (PFM)

and synchronous rectification to create a boost converter

that is both highly efficient and simple to use. A PFM

regulator charges a single inductor for a fixed period of

time and then completely discharges before another cycle

begins, simplifying the design by eliminating the need for

conventional current limiting circuitry. Synchronous

rectification is accomplished by replacing an external

Schottky diode with an on-chip PMOS device, reducing

switching losses and external component count.

REGULATOR OPERATION

A block diagram of the boost converter is shown in Figure

output of amplifier A1 goes high, signaling the regulator

to deliver charge to the output. Since the output of

amplifier A2 is normally high, the flip-flop captures the

A1 set signal and creates a pulse at the gate of the NMOS

transistor Q1. The NMOS transistor will charge the

inductor L1 for 5µs, resulting in a peak current given by:

(1)

does not exceed 1A.

When the one-shot times out, the NMOS transistor

momentarily charge the output through the body diode of

PMOS transistor Q2. But, as the voltage across the PMOS

transistor changes polarity, its gate will be driven low by

the current sense amplifier A2, causing Q2 to short out its

body diode. The inductor then discharges into the load

through Q2. The output of A2 also serves to reset the flip-

flop and one-shot in preparation for the next charging

cycle. A2 releases the gate of Q2 when its current falls to

initiate another pulse. The output capacitor (C1) filters the

inductor current, limiting output voltage ripple. Inductor

current and one-shot waveforms are shown in Figure 3.

RESET COMPARATOR

or any other error condition that is important to the user.

The inverting input of the comparator is internally

provided externally at the DETECT pin. The output of the

GND when an error is detected. (Refer to Block Diagram)

DESIGN CONSIDERATIONS

INDUCTOR

Selecting the proper inductor for a specific application

usually involves a trade-off between efficiency and

maximum output current. Choosing too high a value will

keep the regulator from delivering the required output

current under worst case conditions. Choosing too low a

value causes efficiency to suffer. It is necessary to know

the maximum required output current and the input

voltage range to select the proper inductor value. The

maximum inductor value can be estimated using the

following formula:

(2)

where

h is the efficiency, typically between 0.8 and 0.9.

Note that this is the value of inductance that just barely

delivers the required output current under worst case

conditions. A lower value may be required to cover

inductor tolerance, the effect of lower peak inductor

currents caused by resistive losses, and minimum dead

time between pulses.

Another method of determining the appropriate inductor

value is to make an estimate based on the typical

performance curves given in Figures 4 and 5. Figure 4

shows maximum output current as a function of input

voltage for several inductor values. These are typical

performance curves and leave no margin for inductance

and ON-time variations. To accommodate worst case

conditions, it is necessary to derate these curves by at

least 10% in addition to inductor tolerance.

For example, a two cell to 5V application requires 60mA

of output current while using an inductor with 15%

tolerance. The output current should be derated by 25% to

80mA to cover the combined inductor and ON-time

tolerances. Assuming that 2V is the end of life voltage of

a two cell input, Figure 4 shows that with a 2V input, the

ML4851-5 delivers 80mA with an 18µH inductor.

Figure 5 shows efficiency under the conditions used to

create Figure 4. It can be seen that efficiency is mostly

independent of input voltage and is closely related to

inductor value. This illustrates the need to keep the

inductor value as high as possible to attain peak system

efficiency. As the inductor value goes down to 10µH, the

efficiency drops to around 75%. With 33µH, the

efficiency exceeds 90% and there is little room for

improvement. At values greater than 33µH, the operation

of the synchronous rectifier becomes unreliable at low

input voltages because the inductor current is so small

that it is difficult for the control circuitry to detect. The

data used to generate Figures 4 and 5 is provided in Table

1.

I

tV

L

sV

L

LPEAK

ON

IN

IN

×

=

×

1

5

1

µ

L

Vt

VI

MAX

IN MIN

ON MIN

OUT

OUT MAX

=

××

××

()

()

()

2

2

η