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ML4866IS

3.3V Output DC–DC Step-Down Converter

Micro Linear Corporation 

ML4866

ELECTRICAL CHARACTERISTICS (Continued)

SYMBOL

PARAMETER

SUPPLY

IIN

VIN Current

CONDITIONS

IOUT = 0mA, BURST = 5V

IOUT = 0mA, BURST = 0V

SHDN = 0V

Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.

MIN

TYP

MAX UNITS

400

500

µA

120

220

µA

20

35

µA

FUNCTIONAL DESCRIPTION

The ML4866 is a current-mode, step-down (buck)

converter designed to keep the buck inductor current in

the continuous conduction mode (CCM). Current-mode

operation provides faster output response to input voltage

and output current changes along with cycle-by-cycle

current limiting. CCM inductor current is preferred when

the highest conversion efficiencies are required.

For high efficiencies at low output current, the ML4866

contains an autoburst function which automatically

switches from pulse width modulation (PWM) to pulsed

frequency modulation (PFM) operation when the output

current drops below 100mA. Selection of either mode is

possible by applying the correct logic level signal to the

BURST pin. When operating in PWM mode, loop

compensation of the ML4866 is simplified due to its

transconductance type error amplifier.

An under voltage lockout (UVLO) circuit within the

ML4866 enables the converter when the input voltage

is greater than 3.25V and disables it when the input

voltage is below 3.10V. The IC can also be disabled

externally by applying a logic low signal to the SHDN

pin. When disabled, the ML4866 draws less than 20µA of

current.

The internal 1.25V bandgap reference is made available

via the VREF pin, and may be used for general

applications requiring less than 10µA of current. For

proper operation, this pin must always be bypassed to

GND with a 100nF capacitor.

BURST MODE

Burst (PFM) mode is a method of regulating the output

voltage by applying a variable frequency modulation

technique to the buck inductor. This method maintains

higher efficiencies at light loads than if PWM were used.

If BURST is left open, the ML4866 switches from PWM

mode to PFM mode when the output current falls below

100mA. When the output voltage falls out of regulation

while in PFM mode, the internal buck switch turns on and

ramps the inductor current up to 300mA. The buck switch

then turns off and the synchronous switch turns on,

ramping the inductor current down to 0mA. This action is

repeated until the output voltage returns to its nominal

setting and begins again when the output drops below its

nominal setting. The rate or frequency at which this

“bursting” occurs is directly proportional to the output

current. When the average output current rises above

130mA, the ML4866 returns to PWM operation.

For applications having a load current range of less than

100mA and greater than 130mA, the BURST pin should be

left open and bypassed to ground with a 15nF or larger

capacitor. It is possible to tailor an application for the

highest possible efficiency by externally forcing the

ML4866 into either control mode. Applying a logic low

level to BURST forces the IC into PFM mode. Conversely,

a logic high places it in PWM mode. Care should be

taken to avoid reducing the efficiency by placing the

controller in the least efficient mode for a given output

current.

VIN – VOUT

L

–VOUT

L

∆I

Figure 1. Inductor Current

4

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