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L6917B

Considering the high slope for the load transient, a high switching frequency has to be used. In addition to fast

reaction, this helps in reducing output and input capacitor. Inductance value is also reduced.

A switching frequency of 200kHz for each phase is then considered allowing large bandwidth for the compen-

sation network. It can be considered to use the 5V rail for the power conversion in order to allow compatibility

with standard ATX power supply.

– Current Reading Network and Over Current:

per phase) in the worst case (max mosfet temperature). This because the device limits the valley of the

triangular ripple across the inductors. Considering to sense the output current across the low-side mos-

fet RdsON, STB90NF03L has 6.5m

Ω max at 25°C that becomes 9.1mΩ considering the temperature

variation (+40%); the resulting Tran conductance resistor Rg has to be:

(R3 to R6)

– Droop function Design:

Considering a voltage drop of 100mv at full load, the feedback resistor RFB has to be:

(R7)

– Inductor design:

Each phase has to deliver up to 22.5A; considering a current ripple of 5A (<25%), the resulting induc-

tance value is:

(L1, L2)

– Output Capacitor:

Five Rubycon MBZ (2200

µF / 6.3V / 12mΩ max ESR) has been used implementing a resulting ESR of 2.4mΩ

resulting in an ESR voltage drop of 45A*2.4m

Ω = 108mV after a 45A load transient.

– Compensation Network:

A voltage loop bandwidth of 20kHz is considered to let the device fast react after load transient.

(R8)

(C2)

Rg

I

MAX

2

-------------

RdsO N

35

µ

---------------------

⋅

46

2

------

9.1m

35

µ

-------------

⋅

5.9k

Ω

==

=

R

FB

100m V

70

µA

-------------------

1.43k

Ω

==

L

Vi n

Vo ut

–

l

∆

-----------------------------

d

Fsw

-----------

⋅

12

1.7

–

5

---------------------

1.7

12

--------

1

300000

--------------------

⋅⋅

1

µH

==

=

R

F

R

FB

V

OSC

∆

⋅

V

IN

----------------------------------

5

4

---

ω

T

L

2R

DRO OP

ESR

+

()

⋅

-------------------------------------------------------

⋅⋅

⋅

1.43k 2

⋅

12

----------------------

5

4

--- 20k 2

π

1

µ

2

9.1m

5.9k

------------- 1.43k

2.4m

+

⋅





⋅

-------------------------------------------------------------------

⋅⋅

⋅

⋅

6200

Ω

==

=

C

F

Co

L

2

---

⋅

R

F

--------------------

62200

µ

1

µ

2

-------

⋅⋅

6.2k

-----------------------------------------

15 nF

==

=