CS51312 datasheet(16/18 Pages) CHERRY

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Application Information: continued

and output capacitors and load. This resistor carries the

full load current and should be chosen so that both DC and

AC tolerance limits are met.

In order to determine the droop resistor value the nominal

voltage drop across it at full load has to be calculated. This

voltage drop has to be such that the output voltage at full

load is above the minimum DC tolerance spec:

VDROOP(TYP) =

Current Limit

The current limit setpoint has to be higher than the normal

full load current. Attention has to be paid to the current

rating of the external power components as these are the

first to fail during an overload condition. The MOSFET

continuous and pulsed drain current rating at a given case

temperature has to be accounted for when setting the cur-

rent limit trip point.

Nominal Current Limit Setpoint

From the overcurrent detection data in the electrical char-

acteristics table:

VTH(TYP) = 86mV,

ICL(NOM) =

Design Rules for Using a Droop Resistor

The basic equation for laying an embedded resistor is:

RAR = ρ ×

or R = ρ ×

where

A= W × t = cross-sectional area;

ρ= the copper resistivity (µΩ-mil);

L= length (mils);

W = width (mils);

t = thickness (mils).

An embedded PC trace resistor has the distinct advantage

of near zero cost implementation. However, this droop

resistor can vary due to three reasons: 1) the sheet resistivi-

ty variation caused by variation in the thickness of the PCB

layer; 2) the mismatch of L/W; and 3) temperature varia-

tion.

1) Sheet Resistivity

For one ounce copper, the thickness variation is typically

1.26 mil to 1.48 mil. Therefore the error due to sheet resis-

tivity is:

= ±8%.

2) Mismatch due to L/W

The variation in L/W is governed by variations due to the

PCB manufacturing process. The error due to L/W mis-

match is typically 1%.

3) Thermal Considerations

Due to I2 × R power losses the surface temperature of the

droop resistor will increase causing the resistance to

increase. Also, the ambient temperature variation will con-

tribute to the increase of the resistance, according to the

formula:

R = R20 [1+ α20(Τ−20)],

where

R20 = resistance at 20˚C;

α =;

T= operating temperature;

R = desired droop resistor value.

For temperature T = 50˚C, the % R change = 12%.

Droop Resistor Tolerance

Tolerance due to sheet resistivity variation

±8%

Tolerance due to L/W error

Tolerance due to temperature variation

12%

Total tolerance for droop resistor

21%

Droop Resistor Length, Width, and Thickness

The minimum width and thickness of the droop resistor

should primarily be determined on the basis of the current-

carrying capacity required, and the maximum permissible

droop resistor temperature rise. PCB manufacturer design

charts can be used in determining current- carrying capaci-

ty and sizes of etched copper conductors for various tem-

perature rises above ambient.

Thermal Management

Thermal Considerations for Power MOSFET

In order to maintain good reliability, the junction tempera-

ture of the semiconductor components should be kept to a

maximum of 150°C or lower. The thermal impedance

(junction to ambient) required to meet this requirement

can be calculated as follows:

Thermal Impedance =

A heatsink may be added to TO-220 components to reduce

their thermal impedance. A number of PC board layout

techniques such as thermal vias and additional copper foil

area can be used to improve the power handling capability

of surface mount components.

EMI Management

As a consequence of large currents being turned on and off

at high frequency, switching regulators generate noise as a

consequence of their normal operation. When designing

for compliance with EMI/EMC regulations, additional

components may be added to reduce noise emissions.

These components are not required for regulator operation

and experimental results may allow them to be eliminated.

The input filter inductor may not be required because bulk

TJ(MAX) - TA

Power

0.00393

˚C

1.48 - 1.26

1.37

(W × t)

VTH(TYP)

RSENSE(NOM)

VDAC(MIN)-VDC(MIN)

1+RDROOP(TOLERANCE)

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