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L6918 L6918A

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. Considering the high output current, power conversion will start from the 12V bus.

– Current Reading Network and Over Current:

(27.5A x 4) in the worst case (max mosfet temperature). Since the device limits the valley of the trian-

gular ripple across the inductors, the current ripple must be considered too. Considering the inductor

core saturation, a current ripple of 10A has to be considered so that the OCP threshold in worst case

becomes OCPx = 22A (27.5A-5A). Considering to sense the output current across the low-side mosfets

becomes 9.1m

Ω at 100°C considering the temperature variation; the resulting transconductance resis-

tor Rg has to be:

– Droop function Design:

– Inductor design:

Transient response performance needs a compromise in the inductor choice value: the biggest the in-

ductor, the highest the efficient but the worse the transient response and vice versa. Considering then

an inductor value of 1

µH, the current ripple becomes:

– Output Capacitor:

Ten Rubycon MBZ (3300

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

1.2m

Ω resulting in an ESR voltage drop of 52A*1.2mΩ = 62mV after a 52A load transient.

– Compensation Network:

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

(R8)

(C2)

Further adjustments can be done on the work bench to fit the requirements and to compensate layout parasitic

components.

Rg

I

OCPx

R

dsO N

35

µ

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

⋅

22

4.5m

35

µ

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

⋅

2.7 k

Ω

(R3 to R6; R24 to R27)

==

=

R

FB

85m V

70

µA

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

1.2 k

Ω (R7)

==

I

∆

Vin

Vo ut

–

L

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

d

Fsw

-----------

⋅

12

1.4

–

1

µ

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

1.4

12

--------

1

200k

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

⋅⋅

6.2A

(L1, L2)

==

=

R

F

R

FB

V

OS

∆

⋅

V

IN

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

5

4

---

ω

T

L

2R

D R OOP

ESR

+

()

⋅

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

⋅⋅

⋅

1.2K 2

⋅

12

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

5

4

--- 20k 2

Π

1

µ

2

4.5m

2.7

------------- 1k

1.2m

+

⋅





⋅

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

⋅

⋅

⋅⋅

3.9k

Ω

==

=

C

F

Co

L

2

---

⋅

R

F

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

63300

µ

1

µ

2

-------

⋅

⋅

3.9k

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

22 nF

==

=