10

FN9011.3

To calculate the dissipation in the 5m

Ω resistor, we used

only half of the ripple current, 4A, to give a nominal

dissipation of:

10m

Ω, 1W resistors in parallel were selected.

As discussed in the section under Droop Voltage and shown

transconductance error amplifier. It is this resistor that sets

the droop voltage or regulation. Like any feedback system,

the higher the gain the better the regulation. The value of this

resistor may be determined from the following equation:

of two resistors that form a voltage divider from the internal

3V reference supply.

As described earlier in the Circuit Description section, the

output voltage of the gm amplifier establishes the threshold

voltage of the current comparator. At approximately 1V, the

current comparator threshold voltage is near zero. With no

current demands, the regulator output voltage would be the

same as the programmed DAC voltage. However, an 8A

ripple current was selected for this design. This results in the

output of the gm amplifier moving upwards to supply the

The voltage divider establishes the reference voltage for

amplifier must drive the COMP pin 50mV more positive to

bring it to 1.25V from the 1.2V originally set. This additional

50mV output will result in an input voltage to the error

amplifier of: 50mv / 19.1 = 2.62mV below the programmed

DAC voltage of 1.8V. Neglected, is a negative term

associated with the 60ns delay of the current comparator.

This delay will cause the current ramp to be slightly greater

than predicted by the equation. This means that the initial

setting should be slightly reduced to account for the increase

in current.

that make up the voltage divider must be determined. Figure

8 shows the equations to determine the resistor network that

Optimum transient response depends upon the selection of

the compensation capacitor network placed across the

output of the transconductance error amplifier.

across the error amplifier may be determined by making the

product of the regulator output resistance and output

the equation for the compensation capacitor:

A 1nF capacitor was selected from transient testing. To

prevent excessive phase shift due to the compensation

capacitor, it is usually necessary to place a resistor inseries

with the capacitor to prevent excessive phase shift beyond

the frequency of interest. This is pole cancellation and the

Many variables have been used in the selection of the

various gain and filter networks to this point. A broad range

been used in the design. Therefore, it is important to

evaluate the entire system with dynamic pulse load testing.

This will verify optimum transient response and also indicate

poor response in terms of excessive overshoot, ringing or

oscillation if the compensation network is not optimum.

I

RMS

Ip D

Ip

1.8V

12V

------------

==

Power

0.69W per channel or 1.38W for both channels

=

Power

Ip

2

DR

SENSE

3

=

×

×

=

∴

0.4

2

0.15

·

5m

Ω

×

×

R

L

ni

R

SENSE

×

gm

R

OUT

2

×

×

-----------------------------------------

12.5

5m

Ω

×

2.2mS

1.63m

Ω 2

×

×

--------------------------------------------------------

8.7k

Ω

==

=

gmAmplifierGain

gm

RL

×

2.2mS

8.7k

19.1

=

×

=

=

∴ gm Amplifier Gain =

1V

8A

5m

Ω 12.5

×

×

2

---------------------------------------------

+

=

1V

250mV

+

1.25V

==

V

SET

1V

I

RIPPLE

R

SENSE

ni

×

×

2

----------------------------------------------------------------

+

=

To COMP pin,

R

U

R

B

||

R

L

=

R

U

V

REF

V

SET

---------------

=

R

L

×

R

B

V

SET

V

REF

V

SET

–

------------------------------------

=

R

U

×

V

SET

1V

I

RIPPLE

R

SENSE

ni

×

×

2

----------------------------------------------------------------

+

=

R

U

3V

1.25V

----------------

=

8.7k

×

20.9k

=

R

B

1.25V

3V

1.25V

–

-----------------------------

=

20.9k

×

14.9k

=

C

C

R

OUT

C

OUT

×

R

L

---------------------------------------

1.63m

Ω 9mF

×

8.7k

-----------------------------------------

1.68nF

==

=

RU

AC Equivalent

To COMP pin

FIGURE 9. COMPENSATION CIRCUIT

ISL6560