October 2005

11

MIC2185

MIC2185

Micrel, Inc.

V V

V

I

V

R

R

L

IN

O

O

IN

WINDING

DS(ON)

=

−

×

×

×

+

(

)

η

where:

MOSFET

The maximum value of current sense resistor is:

R

V

I

SENSE

SENSE

IND(pk)

=

where:

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 MIC2185. 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 MIC2185 synchronous boost converter drives both a

high side and low side MOSFET. The low side drive, OUTN,

drives an n-channel MOSFET. The high-side drive, OUTP,

is designed to switch a p-channel MOSFET (the p-channel

MOSFET doesn't require a bootstrap circuit which would

must be connected to the output, which provides power to

drive the high and low side MOSFETs. In skip mode, the

high side MOSFET is disabled by forcing the OUTP pin to

MOSFET Selection

proximately equal to the output voltage. The maximum Vds

rating of the MOSFET must be high enough to allow for ring-

ing and spikes. The MIC2185 input voltage range is 2.9V to

use with this part.

The n-channel gate drive voltage is supplied by the OUTN

output. At startup in a boost converter, the output voltage

n-channel MOSFET must be low enough to operate at the

minimum input voltage to guarantee the boost converter will

start up. The p-channel MOSFET must have a minimum

threshold voltage equal to or lower than the output voltage.

Five volt threshold (logic level) MOSFETs are recommended

for the p-channel MOSFET. Ringing in the gate drive signal

may cause MOSFETs with lower gate thresholds to errone-

ously turn on.

There is a limit to the maximum amount of gate charge the

MIC2185 will drive. Higher gate charge will slow down the

turn-on and turn-off times of the MOSFETs. The MOSFET’s

must be able to completely turn on and off within the driver

non-overlap time or shoot-through will occur.

MOSFET gate charge is also limited by power dissipation in

the MIC2186. The power dissipated by the gate drive circuitry

is calculated below:

n- and p-channel MOSFETs.

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

be driven by the MIC2185 over the input voltage range, for

different values of switching frequency.

0

20

40

60

80

100

120

140

3

5

7

9

11

13

INPUT VOLTAGE (V)

Frequency vs.

Maximum Gate Charge

200kHz

300kHz

400kHz

500kHz

600kHz

Figure 7 - MIC2185 Frequency vs. Max. Gate Charge

External Schottky Diode

An external boost diode in parallel with the high side MOSFET

is used to keep the inductor current flow continuous during

the non-overlap time when both MOSFETs are turned off.

Although the average current through this diode is small,

the diode must be able to handle currents equal to the peak

inductor current. This peak current is calculated in the Cur-

rent Limit section of this specification

The reverse voltage requirement of the diode is:

V

V

DIODE_RRM

OUT

=

For the MIC2185, Schottky diodes with a 30V or 40V rating

are recommended. Schottky diodes with lower reverse volt-

age ratings have higher reverse leakage current which will

cause ringing and excessive power dissipation in the diode

and low side MOSFET.

The external Schottky diode is not necessary for circuit

operation since the high side MOSFET contains a parasitic

body diode. However, the body diode has a relatively slow

reverse recovery time and a relatively high forward voltage

drop. The lower forward voltage drop of the Schottky diode

both prevents the parasitic diode from turning on and im-

proves efficiency. The lack of a reverse recovery mechanism

in a Schottky diode causes less ringing than the MOSFET's

parasitic diode. Depending on the circuit components and

operating conditions, an external Schottky diode will improve