CS5308GDWR28 datasheet(27/31 Pages) ONSEMI

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CS5308

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ICIN,RMS + [2D @ (IC,MIN2 ) IC,MIN @ DIC,IN

(11)

) DIC,IN2 3) ) IIN,AVG2 @ (1 * 2D)]1 2

+ [0.662 @ (3.32 ) 3.3 @ 5 ) 52 3) ) 11.442

@ (1 * 0.662)]1 2

+ 8.94 ARMS

At this point, the designer must decide between saving

board space by using higher−rated/more costly capacitors or

saving cost by using more lower−rated/less costly

capacitors. To save board space, we choose the SP (Oscon)

series capacitors by Sanyo: 680

mF, 6.3 V, 4.84 ARMS, 13 mW,

10 × 10.5mm. We need approximately 8.94 A/4.84 A = 1.84

or NIN = 2 capacitors on the input for a conservative design.

4. Input Inductor Selection

The input inductor must limit the input current slew rate

to less than 0.1 A/

ms during a load transient from 0 to 28 A.

A conservative value will be calculated assuming the

minimum number of output capacitors (NOUT = 5), two

input capacitors (NIN = 2), worst case ESR values for both

the input and output capacitors, and a maximum duty cycle

(D = (1.825 V + 45 mVAVP)/5.0 VIN = 0.374).

First, use Equation 15 to calculate the voltage across the

output inductor due to the 28 A load current being shared

equally between the two phases:

(15)

DVLo + VIN * VOUT,NO−LOAD

) (IO,MAX 2) @ ESROUT NOUT

+ 5.0 V * 1.87 V ) 14 A @ 23 mW 5

+ 3.194 V

Second, use Equation 16 to determine the rate of current

increase in the output inductor:

dILo dt + DVLo Lo

+ 3.194 V 825 nH + 3.872 V ms

(16)

Finally, use Equations 17 and 18 to calculate the minimum

input inductance value:

DVCi + ESRIN NIN @ dILo dt @ D fSW

+ 13 mW 2 @ 3.872 ms @ 0.374 335 kHz

+ 28.1 mV

(17)

LiMIN + DVCi dIIN dtMAX

+ 28.1 mV 0.1 A ms + 281 nH

(18)

We choose the small, cost effective T30−26 core from

Micrometals (33.5 nH/N2) with #16 AWG. We need at least

2.89 or 3 turns to achieve the minimum inductance value.

With three turns the input inductor will be:

Li + 32 @ 33.5 nH N2 + 301 nH

This inductor is available as part number CTX15−14771

from Coiltronics.

Figure 26. CS5308 Circuitry With Only 5 Rubycon

Output Capacitors, 2 Oscon Input Capacitors

and a 300 nH Input Inductor. The dIIN/dt of the

Input Current (0.064 A/ms) Is Much Lower Than

Expected (0.1 A/ms) Because of Input Voltage Drop

and Lower Real ESRs Than Specified in the

Capacitors’ Data Sheets.

5. MOSFET & Heatsink Selection

The NTB75N03−06 from ON Semiconductor is chosen

for both the control and synchronous MOSFET due to its

low RDS(on) and low gate−charge requirements. The

following parameters are derived from the NTB75N03−06

data sheet:

RDS(on) = 5.3 mW

QSWITCH = 29 nC

QG = 52 nC

QRR = 23 nC

QOSS = 35 nC (approx.)

Vfdiode = 0.76 V @ 15 A

qJC = 1.0°C / W

CS5308 Parameters:

iG = 1 A

VG = 10 V

t_nonoverlap = 65 ns

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:

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