MIC2168AYMM datasheet(9/18 Pages) MICREL

9 / 18 page

Micrel, Inc.

MIC2168A

Figure 1. MIC2168A Current Limiting Circuit

The current limiting resistor RCS is calculated by the

following equation:

200µA

DS(ON)Q1

LOAD

where:

IPP = Inductor Ripple Current =

SWITCHING

OUT

−

FSWITCHING = 1MHz

200

μA is the internal sink current to program the

MIC2168A current limit.

The

MOSFET

RDS(ON) varies 30% to 40% with

temperature; therefore, it is recommended to add a 50%

margin to the load current (ILOAD) in the above equation

to avoid false current limiting due to increased MOSFET

junction temperature rise. It is also recommended to

connect RCS resistor directly to the drain of the top

MOSFET Q1, and the RSW resistor to the source of Q1

to accurately sense the MOSFETs RDS(ON). To make the

MIC2168A insensitive to board layout and noise, a 1.4Ω

resistor and a 1000pF capacitor is recommended below

the switch node and ground. A 0.1

μF capacitor in

parallel with RCS should be connected to filter some of

the switching noise.

Internal VDD Supply

The MIC2168A controller internally generates VDD for

self biasing and to provide power to the gate drives. This

VDD supply is generated through a low-dropout regulator

and generates 5V from VIN supply greater than 5V. For

supply voltage less than 5V, the VDD linear regulator is

approximately 200mV in dropout. Therefore, it is

recommended to short the VDD supply to the input

supply through a

10Ω resistor for input supplies between

2.9V to 5V.

MOSFET Gate Drive

The MIC2168A high-side drive circuit is designed to

switch an N-Channel MOSFET. The Functional Diagram

shows a bootstrap circuit, consisting of D1 and CBST,

supplies energy to the high-side drive circuit. Capacitor

CBST is charged while the low-side MOSFET is on and

the voltage on the VSW pin is approximately 0V. When

the high-side MOSFET driver is turned on, energy from

CBST is used to turn the MOSFET on. As the MOSFET

turns on, the voltage on the VSW pin increases to

approximately VIN. Diode D1 is reversed biased and

CBST floats high while continuing to keep the high-side

MOSFET on. When the low-side switch is turned back

on, CBST is recharged through D1. The drive voltage is

derived from the internal 5V VDD bias supply. The

nominal low-side gate drive voltage is 5V and the

nominal high-side gate drive voltage is approximately

4.5V due the voltage drop across D1. An approximate

20ns delay between the high- and low-side driver

transitions

used

prevent

current

from

simultaneously

flowing

unimpeded

through

both

MOSFETs.

MOSFET Selection

The MIC2168A controller works from input voltages of

3V to 13.2V and has an internal 5V regulator to provide

power to turn the external N-Channel power MOSFETs

for high- and low-side switches. For applications where

VIN < 5V, the internal VDD regulator operates in dropout

mode, and it is necessary that the power MOSFETs

used are low threshold and are in full conduction mode

for VGS of 2.5V. For applications when VIN > 5V; logic-

level MOSFETs, whose operation is specified at VGS =

4.5V must be used.

It is important to note the on-resistance of a MOSFET

increases with increasing temperature. A 75°C rise in

junction

temperature

will

increase

the

channel

resistance of the MOSFET by 50% to 75% of the

resistance specified at 25°C. This change in resistance

must be accounted for when calculating MOSFET power

dissipation and in calculating the value of current-sense

(CS) resistor.

April 22, 2015

Revision 3.0

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