July 2002

11

MIC2186

MIC2186

Micrel

R

V

I

SENSE

SENSE

IND(pk)

=

where:

Vsense is the minimum current sense threshold

of the CSH 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:

Io is the maximum output current

Vo is the output voltage

Vin is the minimum input voltage

L is the value of the boost inductor

fs is the switching frequency

η is the efficiency of the boost converter

VL is the voltage across the inductor

VL may be approximated as Vin for higher input voltage.

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 MIC2186

operates at.

VV

VI

V

RR

LIN

OO

IN

WINDING

DSON

=−

×

×

×+

()

η

where:

Rwinding is the winding resistance of the inductor

Rdson is the on resistance of the low side switching

MOSFET

The maximum value of current sense resistor is:

R

V

I

SENSE

SENSE

IND(pk)

=

where:

V

of the CSH pin

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

signal level. The current sense voltage measurement is

referenced to the signal ground pin of the MIC2186. 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 CHS 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

switching 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 MIC2186 converter drives a low side N-channel MOSFET.

The driver for the OutN pin has a 1.6

Ω typical source and sink

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

circuit. It typically connected to the output. The maximum

supply voltage to the VinP pin is 14V. If the output voltage is

greater than 14V or if it is desired to drive the MOSFET with

a voltage less than Vout, the VinP pin can be connected to the

input or to an separate supply voltage.

MOSFET Selection

In a boost converter, the Vds of the MOSFET is approxi-

mately equal to the output voltage. The maximum Vds rating

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

spikes in addition to the output voltage.

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

Vgs threshold voltage of the N-channel MOSFET must be low

enough to operate at the minimum VinP voltage to guarantee

the boost converter will start up.

The maximum amout of MOSFET gate charge that can be

driven is limited by the power dissipation in the MIC2186. The

power dissipated by the gate drive circuitry is calculated

below:

P_gate_drive=Q_gate * VinP * fs

where:

Q_gate is the total gate charge of the external

MOSFET

The graph in Figure 7 shows the total gate charge which can

be driven by the MIC2186 over the input voltage range, for

different values of switching frequency. Higher gate charge

will slow down the turn-on and turn-off times of the MOSFET,

which increases switching losses.

0

20

40

60

80

100

120

140

160

180

200

220

0

2

4

6

8

101214

V

Power Dissipation

vs. Frequency

200kHz

100kHz

400kHz

600kHz

500kHz

Figure 7 - MIC2186 freq vs pdiss

External Schottky Diode

In a boost converter topology, the boost diode, D1 must be

rated to handle the peak and average current. The average

current through the diode is equal to the average output

current of the boost converter. The peak current is calculated

in the current limit section of this specification.