NCP5331 datasheet(33/38 Pages) ONSEMI

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NCP5331

http://onsemi.com

First, use Equation 15 to calculate the voltage across the

output inductor due to the 52 A load current being shared

equally between the two phases.

(15)

DVLo + VIN * VCORE,NO- LOAD

) (IO,MAX 2) @ ESROUT NOUT

+ 12 V * 1.575 V ) 52 A 2 @ 19 mW 6

+ 10.51 V

Second, use Equation 16 to determine the rate of current

increase in the output inductor when the load is applied (i.e.,

Lo has decreased to 88% due to the dc current).

dILo dt + DVLo Lo

+ 10.51 V 729 nH + 14.4 V ms

(16)

Finally, use Equation 17 and Equation 18 to calculate the

minimum input inductance value.

DVCi + ESRIN NIN @ dILo dt @ D fSW

+ 13 mW 5 @ 14.4 V ms @ 0.146 200 kHz

+ 28 mV

(17)

LiMIN + DVCi dIIN dtMAX

+ 28 mV 0.50 A ms + 55 nH

(18)

Next, choose the small, cost effective T30-26 core from

Micrometals (33.5 nH/N2) with #16 AWG. The design

requires only 1.28 turns to achieve the minimum inductance

value. We allow for inductance “swing” at full-load by

using three turns. The input inductor’s value will be

Li + 32 @ 33.5 nH N2 + 301 nH

This inductor is available as part number CTX15-14771

from Coiltronics.

5. MOSFET & Heatsink Selection

For the upper MOSFET we choose two (1) NTD60N03

and for the lower MOSFETs we choose two (2) NTD80N02,

both are from ON Semiconductor. The following parameters

are derived from the data sheets.

NCP5331 Parameter

Value

Gate Drive Current

1.5 A for 1.0

µs

Upper Gate Voltage

6.5 V

Lower Gate Voltage

11.5 V

Gate Nonoverlap Time

65 ns

Parameter

NTD60N03

NTD80N02

RDS(on)

8.0 m

Ω @ 6.5 V

5.0 m

Ω @ 10 V

QSWITCH

27 nC

26 nC

QRR

43 nC

36 nC

QOSS

12 nC

VF,diode

0.75 V @ 2.3 A

0.92 V @ 20 A

θJC

1.65

°C/W

1.65

°C/W

The rms value of the current in the control MOSFET is

calculated from Equation 20 and the previously derived

values for D, ILMAX, and ILMIN at the converter’s maximum

output current.

+ 0.097 @ [(29.62 ) 29.6 @ 22.4 ) 22.42) 3]1 2

(20)

IRMS,CNTL + [D @ (ILo,MAX2 ) ILo,MAX @ ILo,MIN

) ILo,MIN2) 3]1 2

+ 2.53 ARMS

Equation 19 is used to calculate the power dissipation of

the control MOSFET but has been modified for one upper

and two lower MOSFETs.

PD,CONTROL + {(IRMS,CNTL2) @ RDS(on)}

) (ILo,MAX @ Qswitch Ig @ VIN @ fSW)

) (3 @ Qoss 2 @ VIN @ fSW) ) (VIN @ QRR @ fSW)

(19)

+ {2.532 ARMS @ 8.0 mW}

) (29.6 A @ 27 nC 1.5 A @ 12 V @ 200 kHz)

) (3 @ 12 nC 2 @ 12 V @ 200 kHz)

) (12 V @ 43 nC @ 200 kHz)

+ 0.051 W ) 1.28 W ) 0.043 W ) 0.10 W

+ 1.48 W per FET

The rms value of the current in the synchronous MOSFET

is calculated from Equation 27 and the previously derived

values for D, ILo,MAX, and ILo,MIN at the converter’s

maximum output current.

(27)

IRMS,SYNCH + [(1 * D) @

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

+ (1 * 0.097) @ [(29.62 ) 29.6 @ 22.4 ) 22.42) 3]1 2

+ 23.5 ARMS (shared by two synchronous MOSFETs)

Equation 26 is used to calculate the power dissipation of

each synchronous MOSFET. Note: The rms current is

shared by the two lower MOSFETs so the total rms current

is divided by two in the following equation. Also, during the

nonoverlap time, the per-phase current is shared by two

body diodes so the full load current is divided between two

phases and two forward body diodes per phase.

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

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

(26)

+ (23.5 2)2 ARMS @ 5.0 mW

) 0.92 V @ (52 A 2 2) @ 65 ns @ 200 kHz

+ 0.69 W ) 0.16 W + 0.85 W per FET

NCP5331 Datasheet(PDF) 33 Page - ON Semiconductor

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