MIC9130 datasheet(15/19 Pages) MICREL

15 / 19 page

April 2005

M9999-040805

MIC9130

Micrel, Inc.

series resistance is 10K, for a 500kHz switcher, the maximum

series resistance is 2K.

Sensing Current with a Resistor

The fast transition times of the current signal prohibit the use

of inductive resistors. Standard wire wound power resistors

will not work. Carbon composition or metal ﬁlm resistors or

lowinductancepowerresistorsmaybeused. Theovercurrent

range of the power supply and component tolerances must be

considered when selecting the current sense resistor value.

The power supply speciﬁcation may call for an overcurrent

limit, which must be accounted for when selecting the cur-

rent sense resistor value. The relationship between the peak

primary current and the current sense resistor is:

I R

ISNS

ISENSE

ISNS

= ×

where: Ip is the current in the sense resistor

RISENSE is the current sense resistance

IISNS is the current sourced from the ISNS pin

(40µA)

Rf is the series resistor between the ISNS pin and

the current sense resistor.

The current sense resistor must not be too small or the cur-

rent sense signal will be susceptible to noise. If noise is a

problem, the current signal level should be increased.

An example is illustrated below.

The maximum peak current, IPMAX= 1A at 120% overcurrent

and minimum input voltage

The maximum rms current, IRMS=0.65A

The desired current sense signal amplitude is 500mV at 1A

output current.

The current sense resistor value and power dissipation is:

0.5

SENSE

Ω

DISS

RMS

SENSE

0 65

0 5 0 21

A 0.5Ω, non inductive resistor with at least a 1/2W rating

should be selected.

The series resistor is calculated to allow the 500mV-peak

signal to reach 0.82V.

I R

ISNS

ISENSE

ISNS

−

(

)

− ×

(

)

Ω

0 82 1 0 5

10 25

The next lower value of 10kΩ is selected.

The bandwidth of the 10K resistor and the 25pF input capaci-

tance is calculated. The resistor value must be lowered if the

bandwidth is too low for the switching frequency.

kHz

× ×

630

ππ

The maximum switching frequency of this power supply

should be approximately six times less than the BW to pre-

vent current waveform distortion and excessive delays in

the current loop. This limits the switching frequency to the

range of 100kHz.

Sensing Current with a Current Sense Transformer

Athigherpowerlevels,thepowerdissipationinacurrentsense

resistor is excessive. A current sense transformer can be

used to sense the current while minimizing power dissipation.

See Figure 11. The schematic shows the circuitry necessary

when using a current sense transformer. The resistor, R1,

provides a path to reset the current sense transformer. The

resistor, R2, converts the scaled down current to a voltage,

which is sent to the ISNS pin.

OUT

(pin 16)

ISNS

(pin 14)

MIC9130

IPRI

VIN

Current Sense

Transformer

Figure 11

The voltage at the ISNS pin is calculated by:

ISNS

where: IP is the current in the primary of the current sense

transformer

R2 is the current sense resistance at the

secondary of the current sense transformer

N is the turns ratio of the current sense

transformer (N=Nsec/Npri)

IISNS is the current sourced from the ISNS pin

(40µA)

Rf is the series resistor between the ISNS pin and

the current sense resistor.

Current Transformer example:

The maximum peak current, IPMAX= 5A at 120% overcur-

rent and minimum input voltage

The maximum rms current, IRMS = 3.25A

The full 0.82V peak signal a the ISNS input can be used

since very little power is dissipation in the secondary

side sense resistor. The maximum peak to peak volt-

age at the sense pin (pin 14) is 0.82V at the 5A maximum

output current.

The current sense resistor value and power dissipation

is:

SENSE

0 82 100

16 4

× =

Ω

DISS

PRMS

= 





 ×

= 



 ×

3 25

100

16 4 17 4

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