CS5212 datasheet(10/16 Pages) ONSEMI

Nome de Peças	CS5212

Descrição Electrónicos	Low Voltage Synchronous Buck Controller

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Fabricante Electrônico	ONSEMI [ON Semiconductor]

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CS5212

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10

qT t (TJ * TA) PD

where;

θT is the total thermal impedance (θJC + θSA).

θJC is the junction–to–case thermal impedance of the

MOSFET.

θSA is the sink–to–ambient thermal impedance of the

heatsink assuming direct mounting of the MOSFET (no

thermal “pad” is used).

TJ is the specified maximum allowed junction

temperature.

TA is the worst case ambient operating temperature.

For TO–220 and TO–263 packages, standard FR–4

copper clad circuit boards will have approximate thermal

resistances (

θSA) as shown below:

Pad Size

(in2/mm2)

Single–Sided

1 oz. Copper

0.5/323

60–65

°C/W

0.75/484

55–60

°C/W

1.0/645

50–55

°C/W

1.5/968

45–50

°C/W

2.0/1290

38–42

°C/W

2.5/1612

33–37

°C/W

As with any power design, proper laboratory testing

should be performed to insure the design will dissipate the

required power under worst case operating conditions.

Variables considered

during

testing

should

include

maximum

ambient

temperature,

minimum

airflow,

maximum input voltage, maximum loading, and component

variations (i.e. worst case MOSFET RDS(on)). Also, the

inductors and capacitors share the MOSFET’s heatsinks and

will add heat and raise the temperature of the circuit board

and MOSFET. For any new design, its advisable to have as

much heatsink area as possible – all too often new designs

are found to be too hot and require re–design to add

heatsinking.

Compensation Capacitor Selection

The nominal output current capability of the error amp is

30

µA. This current charging the capacitor on the COMP pin

is used as soft start for the converter. The COMP pin is going

to ramp up to a voltage level that is within 70 mV of what

VFFB is going to be when in regulation. This is the voltage

that will determine the soft start. Therefore, the COMP

capacitor can be established by the following relationship:

C

+ 30 mA

soft start

VFFB(REG)

where:

soft start = output ramp–up time

VFFB(REG) = VFFB voltage when in regulation

30

µA = COMP output current, typ.

The COMP output current range is given in the data sheet

and will affect the ramp–up time. The value of the capacitor

on the COMP pin will have an effect on the loop response

and the transient response of the converter. Transient

response can be enhanced by the addition of a parallel

combination of a resistor and capacitor between the COMP

pin and the comp capacitor.

ROSC Selection

The switching frequency is programmed by selecting the

resistor connected between the ROSC pin and SGND (pin 7).

The grounded side of this resistor should be directly

connected to the SGND pin, without any other currents

flowing between the bottom of the resistor and the pin. Also,

avoid running any noisy signals under the resistor, since

injected noise could cause frequency jitter. The graph in

Figure NO TAG shows the required resistance to program

the frequency. Below 500 kHz, the following formula is

accurate:

R

+ 17544 fSW * 4kW

where fSW is the switching frequency in kHz.

Figure 6. Frequency vs. ROSC

0

Frequency (kHz)

20

40

60

80

100

120

140

100

200

300

400

500

600

700

800

Differential Remote Sense Operation

The ability to implement fully differential remote sense is

provided by the CS5212. The positive remote sense is

implemented by bringing the output remote sense

connection to the positive load connection. A low value

resistor is connected from Vout to the feedback point at the

regulator to provide feedback in the instance when the

remote sense point is not connected.

The negative remote sense connection is provided by

connecting the SGND of the CS5212 to the negative of the

load return. Again, a low value resistor should be connected

between SGND and LGND at the regulator to provide

feedback in the instance when the remote sense point is not

connected. The maximum voltage differential between the

three grounds for this part is 200 mV.

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