MIC23153 datasheet(13/18 Pages) MICREL

13 / 18 page

Micrel Inc.

MIC23153

September 2011

M9999-092211-B

HyperLight Load™ Mode

As shown in the previous equation, the load at which the

MIC23153 transitions from HyperLight Load™ mode to

PWM mode is a function of the input voltage (VIN), output

voltage (VOUT), duty cycle (D), inductance (L) and

frequency (f). As shown in Figure 3, as the Output

Current increases, the switching frequency also

increases until the MIC23153 goes from HyperLight

Load™ mode to PWM mode at approximately 120mA.

The MIC23153 will switch at a relatively constant

frequency around 4MHz once the output current is over

120mA.

MIC23153 uses a minimum on and off time proprietary

control loop (patented by Micrel). When the output

voltage falls below the regulation threshold, the error

comparator begins a switching cycle that turns the

PMOS on and keeps it on for the duration of the

minimum-on-time. This increases the output voltage. If

the output voltage is over the regulation threshold, then

the error comparator turns the PMOS off for a minimum-

off-time until the output drops below the threshold. The

NMOS acts as an ideal rectifier that conducts when the

PMOS is off. Using a NMOS switch instead of a diode

allows for lower voltage drop across the switching device

when it is on. The asynchronous switching combination

between the PMOS and the NMOS allows the control

loop to work in discontinuous mode for light load

operations. In discontinuous mode, the MIC23153 works

in pulse frequency modulation (PFM) to regulate the

output. As the output current increases, the off-time

decreases, thus provides more energy to the output.

This switching scheme improves the efficiency of

MIC23153 during light load currents by only switching

when it is needed. As the load current increases, the

MIC23153 goes into continuous conduction mode (CCM)

and switches at a frequency centered at 4MHz. The

equation to calculate the load when the MIC23153 goes

into continuous conduction mode may be approximated

by the following formula:

Switching Frequency

v s. Load Current

0.1

100

1000

10000

0.0001 0.001

0.01

0.1

LO AD CURRENT (A)

L = 2.2µH

L = 1µH

VOUT = 1.8V

Figure 3. SW Frequency vs. Output Current

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

)

OUT

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