MIC2196

Micrel

MIC2196

8

August 2004

The mode of operation (continuous or discontinuous), the

minimum input voltage, maximum output power and the

minimum value of the current limit threshold determine the

value of the current sense resistor. Discontinuous mode is

where all the energy in the inductor is delivered to the output

at each switching cycle. Continuous mode of operation

occurs when current always flows in the inductor, during both

the low-side MOSFET on and off times. The equations below

will help to determine the current sense resistor value for

each operating mode.

The critical value of output current in a boost converter is

calculated below. The operating mode is discontinuous if the

output current is below this value and is continuous if above

this value.

I

VV

V

2fs L

V

CRIT

IN

2

O

IN

O

2

=

×−

×× ×

η

where:

η is the efficiency of the boost converter

V

L is the value of the boost inductor

F

V

Maximum Peak Current in Discontinuous Mode:

The peak inductor current is:

I

2I

V

V

Lfs

IND(pk)

OO

IN

=

××

− ×

()

×

η

where:

I

V

V

L is the value of the boost inductor

f

η is the efficiency of the boost converter

The maximum value of current sense resistor is:

R

V

I

SENSE

SENSE

IND(pk)

=

where:

V is the minimum current sense threshold of the

CS pin.

Maximum Peak Current in Continuous Mode:

The peak inductor current is equal to the average inductor

current plus one half of the peak to peak inductor current.

The peak inductor current is:

II

1

2

I

I

VI

V

VV

V

2

V

fs L

IND(pk)

IND(ave)

IND(pp)

IND(pk)

OO

IN

L

O

IN

O

=+ ×

=

×

×

+

×−

×

()

×× ×

η

η

where:

I

V

V

L is the value of the boost inductor

f

η is the efficiency of the boost converter

V

V

However, the voltage drop across the inductor winding resis-

tance and low-side MOSFET on-resistance must be ac-

counted for at the lower input voltages that the MIC2196

operates at:

VV

VI

V

RR

LIN

OO

IN

WINDING

DSON

=−

×

×

×+

()

η

where:

R

R

MOSFET

The maximum value of current sense resistor is:

R

V

I

SENSE

SENSE

IND(pk)

=

where:

V

of the CS pin.

The current sense pin, CS, is noise sensitive due to the low

signal level. The current sense voltage measurement is

referenced to the signal ground pin of the MIC2196. The

current sense resistor ground should be located close to the

IC ground. Make sure there are no high currents flowing in this

trace. The PCB trace between the high side of the current

sense resistor and the CS pin should also be short and routed

close to the ground connection. The input to the internal

current sense amplifier has a 30ns dead time at the beginning

of each switching cycle. This dead time prevents leading

edge current spikes from prematurely terminating the switch-

ing cycle. A small RC filter between the current sense pin and

current sense resistor may help to attenuate larger switching

spikes or high frequency switching noise. Adding the filter

slows down the current sense signal, which has the effect of

slightly raising the overcurrent limit threshold.

MOSFET Gate Drive

The MIC2196 converter drives a low-side N-channel MOSFET.

The driver for the OUTN pin has a 2

Ω typical source and sink

impedance. The VIN pin is the supply pin for the gate drive

circuit. The maximum supply voltage to the VIN pin is 14V.

MOSFET Selection

In a boost converter, the V

mately equal to the output voltage. The maximum V

of the MOSFET must be high enough to allow for ringing and

spikes in addition to the output voltage.

The VIN pin supplies the N-channel gate drive voltage. The

V