CS5422

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14

Control IC Power Dissipation

The power dissipation of the IC varies with the MOSFETs

average MOSFET gate charge current typically dominates

the control IC power dissipation.

The IC power dissipation is determined by the formula:

) PGATE(L)1 ) PGATE(H)2 ) PGATE(L)2

PCONTROL(IC) + ICC1VCC1 ) IBSTVBST ) PGATE(H)1

where:

The upper (switching) MOSFET gate driver (IC) losses

are:

PGATE(H) + QGATE(H)

fSW

VBST

where:

The lower (synchronous) MOSFET gate driver (IC)

losses are:

PGATE(L) + QGATE(L)

fSW

VCC

where:

The junction temperature of the control IC is primarily a

function of the PCB layout, since most of the heat is removed

through the traces connected to the pins of the IC.

Current Sensing

The current supplied to the load can be sensed easily using

the IS+ and IS− pins for the output. These pins sense a

voltage, proportional to the output current, and compare it to

a fixed internal voltage threshold. When the differential

voltage exceeds 70 mV, the internal overcurrrent protection

system goes into hiccup mode. Two methods for sensing the

current are available.

Sense Resistor.

A sense resistor can be added in series

with the inductor. When the voltage drop across the sense

resistor exceeds the internal voltage threshold of 70 mV, a

fault condition is set.

The sense resistor is selected according to:

RSENSE + 0.070 V

ILIMIT

In a high current supply, the sense resistor will be a very

low value, typically less than 10 mΩ. Such a resistor can be

either a discrete component or a PCB trace. The resistance

value of a discrete component can be more precise than a

PCB trace, but the cost is also greater.

Setting the current limit using an external sense resistor is

very precise because all the values can be designed to

specific tolerances. However, the disadvantage of using a

sense resistor is its additional constant power loss and heat

generation.

Inductor ESR.

Another means of sensing current is to use

the intrinsic resistance of the inductor. A model of an

inductor reveals that the windings of an inductor have an

effective series resistance (ESR).

The voltage drop across the inductor ESR can be

measured with a simple parallel circuit: an RC integrator. If

L

ESR +

RS1C

then the voltage measured across the capacitor C will be:

VC + ESR

ILIM

Selecting Components.

Select the capacitor C first. A

selected according to:

RS1 +

1

ESR

C

Typical values for inductor ESR range in the low m;

consult manufacturer’s datasheet for specific details.

Selection of components at these values will result in a

current limit of:

ILIM + 0.070 V

ESR

Figure 9. Inductor ESR Current Sensing

GATE(H)

Co

GATE(L)

IS+

IS−

RS1

C

ESR

L

Given an ESR value of 3.5 mΩ, the current limit becomes

20 A. If an increased current limit is required, a resistor

divider can be added.

The advantages of setting the current limit by using the

winding resistance of the inductor are that efficiency is

maximized and heat generation is minimized. The tolerance

of the inductor ESR must be factored into the design of the

current limit. Finally, one or two more components are

required for this approach than with resistor sensing.

Adding External Slope Compensation

Today’s voltage regulators are expected to meet very

stringent load transient requirements. One of the key factors

in achieving tight dynamic voltage regulation is low ESR.

Low ESR at the regulator output results in low output

voltage ripple. The consequence is, however, that very little

resulting in increased regulator sensitivity to noise and the

potential for loop instability. In applications where the