MIC9130 datasheet(16/19 Pages) MICREL

16 / 19 page

MIC9130

Micrel, Inc.

M9999-040805

April 2005

A 16.2 ohm, 1%, non inductive resistor with at least a 50mW

rating should be selected. A good choice would be an 0805

size metal ﬁlm or a 1/8 watt leaded metal ﬁlm resistor. A

series resistor between the current sense transformer and

the Isns input is not necessary unless it is used for low pass

ﬁltering.

If the current sense transformer were not used, the sense

resistor would dissipate 1.7 watts.

SENSE

Ω

0 82

0 164

DISS

RMS

SENSE

3 25

0 164 1 7

Slope Compensation

Power supplies using peak current mode control techniques

require slope compensation when they are operating in

continuous mode and have a duty cycle greater than 50%.

Without slope compensation, the duty cycle of the power sup-

ply will alternate wide and narrow pulses commonly referred

to as subharmonic oscillations. Even though the MIC9130

operates below a 50% duty cycle, slope compensation adds

the beneﬁts of improved transient response and greater

noise immunity in the current sense loop (especially when

the current ramp is shallow). Slope compensation can be

implemented by adding an optimum 1/2 of the inductor cur-

rent downslope, reﬂected back to the current sense input. In

real world applications, 2/3 of the inductor current downslope

is used to allow for component tolerances.

Slope compensation at the ISNS input may be implemented

by using a resistor and capacitor as shown in Figure 12. The

rectangular waveshape of the gate drive output is integrated

by the resistor/capacitor ﬁlter, which results in a ramp used

for the slope compensation signal. When the gate drive and

the current signal at the sense resistor goes low, the capaci-

tor is discharged to 0V.

Gate Drive

(pin 16)

ISNS

(pin 14)

MIC9130

RSENSE

Figure 12

The procedure outlined below demonstrates how to calculate

the component values.

Compute the inductor current downslope as seen at the cur-

rent sense input.

For a ﬂyback, buck or forward mode topology the

inductor downslope is equal to:

2 =

where :

VO is the output voltage

VD is the forward voltage drop of the rectiﬁer diode

L is the inductance of the output inductor (or the

secondary winding inductance for the ﬂyback

topology)

M2 is the inductor current downslope

For a boost topology, the inductor downslope is:

OUT

2 =

−

In a transformer isolated topology, the downslope must be

reﬂected back to the primary by the turns ratio of the trans-

former. The reﬂected downslope is:

REFLECTED

where : Ns/Np is the turns ratio of the secondary winding

to the primary winding.

M2REFLECTED is the inductor curent downslope

reﬂected to the secondary side of the current

sense transformer.

The reﬂected downslope is multiplied by the current sense

resistor to obtain the downslope at the current sense input

pin (ISNS).

SNS SLOPE

REFLECTED

where Rs is the value of the current sense resistor.

The required downslope of the compensation ramp at the

ISNS input is:

ISNS SLOPE

0 67

R1 is know if a value for the resistor between the current

sense resistor and the Isns pin, has already been selected.

If not chose a value of 1k, which will minimize any offset

and signal degradation at the ISNS pin. Select a value of

C1 to minimize signal degradation from the cutoff frequency

of R1/C1. The bandwidth should be at least six times the

switching frequency.

× × ×

ππ

where: fS is the switching frequency of the power

supply (not the oscillator frequency)

The slope of the generated compensation ramp is:

R2 R1

R2 C1

GATE_DRIVE

3 =

Solving for R2 and assuming R2 is much greater than R1.

GATE DRIVE

where: VGATE_DRIVE is the amplitude of the gate

drive waveform

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