MIC24085AYML datasheet(9/16 Pages) MICREL

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Micrel, Inc.

MIC24085

October 22, 2013

Revision 1.1

Functional Description

Micrel’s MIC24085 is an 18V, 3A, 1MHz buck regulator

with

integrated

MOSFETSs

for

low-cost

high-

performance applications. The MIC24085 provides basic

features including enable, power good, internal soft-start,

current-limit, and overtemperature protections.

VCC Supply Voltage and UVLO

The MIC24085 is held in the off state until VCC reaches

the UVLO threshold, which has a 650mV hysteresis. A

ceramic capacitor from VCC to PGND is recommended

for clean operation.

Enable and Soft-Start

When the MIC24085 is enabled, the reference voltage is

controlled to ramp-up from 0V to 0.9V in approximately

4ms. During the soft-start, the current limit is enabled to

protect the device in case of a short circuit present at the

output.

Peak Current-Mode PWM

To achieve fast transient response, the peak current-

mode PWM control is employed in the MIC24085. As

illustrated in the Functional Diagram, the high-side

MOSFET current (ISW) is sensed and summed with a

slope-compensating ramp. Then it is compared with the

error amplifier output to generate a PWM signal.

The loop compensation is designed internally in the

MIC24085 to ensure loop stability, even with a low-ESR

capacitor at the output, such as a ceramic output

capacitor.

PFM Mode at Light Load

The MIC24085 runs in asynchronous buck mode at light

load, where the bottom MOSFET is turned off when the

inductor current drops to zero. This scheme prevents the

output energy from being pumped back to the input, and

improves the over-all efficiency.

Current Limit and Frequency Foldback

In the MIC24085 ISW is also used to determine over-

current conditions. As the peak inductor current is limited,

the output voltage of the buck converter drops as the load

current increases. When the output voltage reaches 60%

of regulated voltage (or feedback voltage reaches 60% of

0.9V), the switching frequency is forced to fold back, from

1MHz to 250kHz. This current-limit scheme reduces the

average inductor current (even with the same peak

inductor current), and improves the power consumption

at the overload conditions.

The current limit is always active when the regulator is

enabled. Once the overcurrent condition is removed, the

device runs to a soft-start and returns to a normal

operation.

Power Good (PG)

Power good (PG) is an open drain output pin. A 10kΩ

resistor to VOUT is required for pull-up. The PG pin turns

low if the output voltage is below 93% of the reference

voltage (0.9V). After VOUT fault and recovery, PG is held

low for a typical duration of 11ms. See the Electrical

Characteristics table for additional information.

Channel 1: VOUT, Channel 2: PG, Hold Time: 11ms

Output Voltage Protection

The MIC24085 turns both high-side and low-side

MOSFETs off to shut down the buck regulator output

voltage when the output voltage exceeds 103% of the

regulated voltage. After the output voltage drops back

below the threshold, the MIC24085 runs to a soft-start.

Thermal Shutdown

The MIC24085 turns both high-side and low-side

MOSFETs off to shut down the buck regulator output

voltage when the internal temperature is over the setting

threshold at around 170°C. After the temperature drops

back below the threshold, the MIC24085 runs to a soft-

start.

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