L6918D 查看數據表（PDF） - STMicroelectronics
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L6918D Datasheet PDF : 35 Pages
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L6918 L6918A
CPU Power Supply: 12VIN; 1.45VOUT; 110ADC
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:
Since the maximum output current is IMAX = 110A, the over current threshold has been set to 110A
(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 RdsON), each STB90NF03L has 6.5mΩ max at 25°C that
becomes 9.1mΩ at 100°C considering the temperature variation; the resulting transconductance resis-
tor Rg has to be:
Rg = IOCPx ⋅ R----3-d---5s---Oµ----N-- = 22 ⋅ 4--3--.--55---µm---- = 2.7 kΩ (R3 to R6; R24 to R27)
– Droop function Design:
Considering a voltage drop of 85mV at full load, the feedback resistor RFB has to be:
RFB = 8-7---50----mµ----A-V--- = 1.2 kΩ (R7)
– 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 1µH, the current ripple becomes:
∆I = V-----i--n-----–-L---V----o----u----t ⋅ F-----sd---w--- = 1----2---1--–--µ--1---.--4-- ⋅ 1-1---.-24-- ⋅ 2----0--1-0----k- = 6.2A (L1, L2)
– Output Capacitor:
Ten Rubycon MBZ (3300µF / 6.3V / 12mΩ max ESR) has been used implementing a resulting ESR of
1.2mΩ 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.
The RF CF network results:
RF
=
R-----F---B---V--⋅--I-∆-N---V----O----S-- ⋅ 54-- ⋅ ωT ⋅ 2-----⋅---(---R----D----R----O----LO----P----+-----E-----S----R-----)-
=
1----.-2---1-K--2----⋅---2--
⋅
54--
⋅
20
k
⋅
2
Π
⋅
--------------------------1----µ----------------------------
2
⋅
4---2-.-5--.-7-m----
⋅
1k
+
1.2 m
=
3.9 k Ω
(R8)
CF
=
-----C----o-----⋅---L2----
RF
=
-----6-----⋅---3--3-3---.0-9---0-k--µ-----⋅----1---2----µ----
=
22 n F
(C2)
Further adjustments can be done on the work bench to fit the requirements and to compensate layout parasitic
components.
31/35
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