L6918A STMicroelectronics, L6918A Datasheet - Page 31

• Manufacturer Part Number: L6918A
• Description: 5 BIT PROGRAMMABLE MULTIPHASE CONTROLLER
• Manufacturer: STMicroelectronics

CPU Power Supply: 12V
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:
– Droop function Design:
– Inductor design:
– Output Capacitor:
– Compensation Network:
Further adjustments can be done on the work bench to fit the requirements and to compensate layout parasitic
components.
Since the maximum output current is I
(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
RdsON (two in parallel to reduce equivalent R
becomes 9.1m
tor Rg has to be:
Considering a voltage drop of 85mV at full load, the feedback resistor R
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 H, the current ripple becomes:
Ten Rubycon MBZ (3300 F / 6.3V / 12m
1.2m
A voltage loop bandwidth of 20kHz is considered to let the device fast react after load transient.
The R
R
F
=
R
----------------------------- -
F
F B
resulting in an ESR voltage drop of 52A*1.2m
C
V
F
IN
network results:
V
O S
Rg
at 100°C considering the temperature variation; the resulting transconductance resis-
5
-- -
4
=
I
=
I
T
OCPx
IN
Vin Vo ut
---------------------------- -
; 1.45V
------------------------------------------------------ -
2
C
–
R
F
L
R
----------------- -
DROOP
=
35
d sO N
OUT
------------------- -
L
C o
; 110A
R
R
+
-----------
Fsw
FB
F
=
ESR
d
L
-- -
2
MAX
22
=
DC
=
=
85mV
--------------- -
70 A
max ESR) has been used implementing a resulting ESR of
4.5m
-------------
=
= 110A, the over current threshold has been set to 110A
---------------------------------------- -
35
12 1.4
-------------------- -
6 3300
1.2K 2
------------------- -
dsON
1
–
12
3.9k
=
=
), each STB90NF03L has 6.5m
1.2 k
2.7 k
1.4
------- -
5
-- - 20k 2
4
12
= 62mV after a 52A load transient.
1
------ -
2
------------ -
200k
(R7)
1
=
(R3 to R6; R24 to R27)
22 nF
=
----------------------------------------------------------
2
6.2A (L1, L2)
4.5m
------------- 1k
(C2)
2.7
FB
1
has to be:
+
1.2m
max at 25°C that
L6918 L6918A
=
3.9k
(R8)
31/35