MIC5167 datasheet(14/23 Pages) MICREL

14 / 23 page

Micrel, Inc.

MIC5167

January 2012

M9999-012312-B

From that, the loss in efficiency due to inductor

resistance can be calculated as follows:

()

100

Loss

Efficiency

OUT

⎥

⎦

⎤

⎢

⎣

⎡

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

Efficiency loss due to DCR is minimal at light loads and

gains significance as the load is increased. Inductor

selection becomes a trade-off between efficiency and size

in this case.

Alternatively, under lighter loads, the ripple current due to

the inductance becomes a significant factor. When light

load efficiencies become more critical, a larger inductor

value maybe desired. Larger inductances reduce the peak-

to-peak inductor ripple current, which minimize losses.

Compensation

The MIC5167 has a combination of internal and external

stability compensation to simplify the circuit for small,

high efficiency designs. In such designs, voltage mode

conversion is often the optimum solution. Voltage mode

is achieved by creating an internal 1MHz ramp signal

and using the output of the error amplifier to modulate

the pulse width of the switch node, thereby maintaining

output voltage regulation. With a typical gain bandwidth

of 100-200kHz, the MIC5167 is capable of extremely fast

transient responses.

The MIC5167 is designed to be stable with a typical

application using a 1µH inductor and a 100µF ceramic

(X5R) output capacitor. These values can be varied

dependant upon the tradeoff between size, cost and

efficiency,

keeping

the

natural

frequency

(

) ideally less than 26kHz to ensure

stability can be achieved. The minimum recommended

inductor value is 0.4µH and minimum recommended

output capacitor value is 100µF. The tradeoff between

changing these values is that with a larger inductor,

there is a reduced peak-to-peak current which yields a

greater efficiency at lighter loads. A larger output

capacitor will improve transient response by providing a

larger hold up reservoir of energy to the output.

Current Limit

The MIC5167 is protected against overload in two

stages. The first is to limit the current in the P-channel

switch; the second is over temperature shutdown.

Current is limited by measuring the current through the

high-side MOSFET during its power stroke and

immediately switching off the driver when the preset limit

is exceeded.

The circuit in Figure 5 describes the operation of the

current limit circuit. Since the actual RDS(ON) of the P-

Channel MOSFET varies part-to-part, over temperature

and with input voltage, simple IR voltage detection is not

employed. Instead, a smaller copy of the Power

MOSFET (Reference FET) is fed with a constant current

which is a directly proportional to the factory set current

limit. This sets the current limit as a current ratio and

thus, is not dependant upon the RDS(ON) value. Current

limit is set to nominal value. Variations in the scale factor

K between the Power PFET and the reference PFET

used to generate the limit threshold account for a

relatively small inaccuracy.

PVIN

I LIMIT/K

CONTROL

LOGIC

CLOCK

PVIN

HSD

LSD

PGND

Figure 5. Current-Limit Detail

Thermal Considerations

The MIC5167 is packaged in a MLF

® 4mm x 4mm, a

package that has excellent thermal performance. This

maximizes heat transfer from the junction to the exposed

pad (ePAD) which connects to the ground plane. The

size of the ground plane attached to the exposed pad

determines the overall thermal resistance from the

junction to the ambient air surrounding the printed circuit

board.

Sequencing and Tracking

The MIC5167 provides PG and EN/DLY pins to provide

sequencing and turn on delay capability for connecting

multiple voltage regulators together.

Enable/DLY Pin

The Enable pin contains a trimmed, 1µA current source

which can be used with a capacitor to implement a fixed

desired delay in some sequenced power systems. The

threshold level for power on is 1.24V with a hysteresis of

20mV.

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