CS5308 datasheet(21/31 Pages) ONSEMI

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CS5308

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QG is the MOSFET total gate charge to obtain RDS(on).

Commonly specified in the data sheet.

Vg is the gate drive voltage.

QRR is the reverse recovery charge of the lower MOSFET.

Qoss is the MOSFET output charge specified in the data

sheet.

For the lower or synchronous MOSFET, the power

dissipation can be approximated from:

PD,SYNCH + (IRMS,SYNCH2 @ RDS(on))

) (Vfdiode @ IO,MAX 2 @ t_nonoverlap @ fSW)

(26)

The first term represents the conduction or IR losses when

the MOSFET is ON and the second term represents the diode

losses that occur during the gate non−overlap time.

All terms were defined in the previous discussion for the

control MOSFET with the exception of:

(27)

IRMS,SYNCH + 1 * D

@ [(ILo,MAX2 ) ILo,MAX @ ILo,MIN ) ILo,MIN2) 3]1 2

where:

Vfdiode is the forward voltage of the MOSFET’s intrinsic

diode at the converter output current.

t_nonoverlap is the non−overlap time between the upper

and lower gate drivers to prevent cross conduction.

This time is usually specified in the data sheet for the

control IC.

−

RCS1

CS1

CCS1

0 A

GVDRP

−

RCS2

CS2

CCS2

0 A

GVDRP

CSREF

COMP

Error

Amp

VID Setting

IBIASVFB

RDRP

RVFBK

VDRP = VID

VFB = VID

VCORE

IDRP = 0

IFBK = IBIASVFB

VCORE = VID + IBIASVFB w RVFBk

Figure 17. AVP Circuitry at No−Load

+ −

When the MOSFET power dissipations are known, the

designer can calculate the required thermal impedance to

maintain a specified junction temperature at the worst case

ambient operating temperature

qT t (TJ * TA) PD

(28)

where:

qT is the total thermal impedance (qJC + qSA);

qJC is the junction−to−case thermal impedance of the

MOSFET;

qSA 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 (

qSA) as shown below:

Pad Size

(in2/mm2)

Single−Sided

1 oz. Copper

0.50/323

60−65°C/W

0.75/484

55−60°C/W

1.00/645

50−55°C/W

1.50/968

45−50°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

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