MIC9131

Micrel, Inc.

M9999-080206

14

August 2006

Gate Drive

Output

GND

T1

1:N

R1

R2

D2

D1

C1

Figure 10

The gate impedance of a MOSFET is capacitive and the

power required to drive the gate is proportional to the charge

required to turn on the MOSFET, the peak gate voltage and

the switching frequency. Assuming the total gate charge for

turn on and turn off is equal, the power used to switch the

MOSFET on and off is:

P

Q

V

f

DRIVE

G

GS

S

=

×

×

The power required to drive the MOSFET is dissipated in the

drive circuitry of the MIC9131. This power must not cause

the die temperature to exceed the maximum rated junction

temperature of 125°C.

MOSFET Driver IC’s are used when the drive requirement for

the MOSFETs is greater than the capability of the MIC9131

gate drive output. While the peak current of the MIC9131

will sink or source between 1.2A and 12A of peak current.

The higher peak current allows faster rise and fall times for

larger MOSFETs.

The drive requirements for selecting a MOSFET driver are

determined using the following equation:

I

Q

t

PK

G

= ×

2

information is usually given in the MOSFET

specification sheet.

t is the gate voltage transition time (risetime or fall

time)

MOSFET driver IC.

it is desired to have a 50nS gate to source risetime/falltime,

the peak current requirement of the MOSFET driver is:

I

nC

ns

A

×

=

2 60

50

2 4

.

A driver such as the MIC4424 will meet this requirement.

For more information on choosing a MOSFET driver, see

the Micrel application note AN-24, “Designing with Low Side

MOSFET Drivers.”

Current Sense Circuit

The current sense input of the MIC9131 has three unique

features, which are advantageous in a high speed, high ef-

ficiency power supply.

1. The overcurrent threshold is nominally 0.82V instead

of the typical 1.0V found in most switching control

ICs.

2. The current sense pin sources a nominal 40µA of

current out of the pin. This is used to raise the current

limit threshold of the pin, which allows a smaller

current sense resistor to be used. This improves the

efficiency of the power supply, especially in lower

current applications.

3. The delay from the current sense input to the output

is typically 50ns.

The current limit threshold of the ISNS pin was set at 0.82V,

allowing the use of a smaller current sense resistor. A stable,

bandgap derived 40µA current is sourced from the ISNS pin.

A voltage drop across a series resistor placed between the

pin and the current sense resistor level increases the current

sense signal at the ISNS pin. This allows the use of a smaller

current sense resistor if the full 0.82V peak to peak current

signal is not required. Decreasing the value of the current

sense resistor decreases the power dissipation in the resistor,

which improves the efficiency of the power supply.

The delay between the input of the overcurrent comparator

and the output gate drive is nominally 50ns. This very fast

response time allows the MIC9131 to operate at higher fre-

quencies and still have adequate overcurrent protection.

The operation of the current sense input is as follows. The

sensed current in the power supply is converted to a volt-

age by a resistor or current sense transformer. Referring to

Figure 1, this voltage is compared to the output of the error

amplifier, which sets the duty cycle of the gate drive output.

The current signal is also connected to an Imax comparator.

Comparing the current sense signal to the reference voltage

sets a maximum current limit. If the maximum amplitude of the

current sense signal exceeds the reference, the comparator

terminates the gate drive output pulse. It aslo discharges the

soft start capacitor when the CPWR pin is high.

Leading Edge Current Spike

The current signal in a power circuit will often have a leading

edge spike caused by leakage inductance, parasitic induc-

tance and capacitance, diode reverse recovery effects and

snubbers. These spikes can cause premature termination of

the switching cycle if they are not eliminated.

A resistor may be added in series between the current sense

resistor and the Isns input. The input and board trace ca-

pacitance of the ISNS pin (pin 14) is approximately 25pF. A

1k resistor is a good choice, since it attenuates most of the

ripple without distorting the current sense waveform. It has

a minimal effect on level, offsetting the current sense signal

by only 40mV.

A typical rule of thumb is the bandwidth of the RC filter

should be at least 6 times the switching frequency. This

avoids distorting the current sense waveform and adding

excessive delays in the current loop that will interfering with

overcurrent protection. For a 100kHz switcher, the maximum