L6919E View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6919E' PDF : 33 Pages View PDF
L6919E
CPU Power Supply: 5 to 12VIN; 1.2VOUT; 45ADC
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 = 45A, the over current threshold has been set to 45A (22.5A
x 2)in the worst case (max mosfet temperature). Since the device limits the valley of the triangular ripple
across the inductors, the current ripple must be considered too. Considering the inductor core satura-
tion, a current ripple of 10A has to be considered so that the OCP threshold in worst case becomes
OCPx=17A (22.5A-5A). Considering to sense the output current across the low-side mosfet RdsON,
SUB85N03L-04P has 4.3mΩ max at 25°C that becomes 5.6mΩ at 100ºC considering the temperature
variation; the resulting transconductance resistor Rg has to be:
Rg
=
IOCPx
⋅
R-----d---s---O----N--
35 µ
=
17 ⋅ 5--3--.--56---mµ----
=
2.7 k Ω
(R3 to R6)
– Droop function Design:
Considering a voltage drop of 70mV at full load, the feedback resistor RFB has to be:
RFB = 7-7---0-0---mµ----A-V--- = 1kΩ (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 0.8µH, the current ripple becomes:
∆I
=
V-----i--n-----–----V-----o----u---t
L
⋅
-----d------
Fsw
=
1----2-----–----1----.-2--
0.8µ
⋅
1----.--2-
12
⋅
------1------
200k
=
6.5A
(L1, L2)
– Output Capacitor:
Five Rubycon MBZ (2200µF / 6.3V / 12mΩ max ESR) has been used implementing a resulting ESR of
2.4mΩ resulting in an ESR voltage drop of 45A · 2.4mΩ = 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.
The RF CF network results:
RF
=
R-----F---B-----⋅---∆----V----O----S--
VIN
⋅
5--
4
⋅
ωT
⋅
---------------------------L----------------------------
2 ⋅ (RDROOP + ESR)
=
1----K------⋅---2--
12
⋅
5--
4
⋅
20
K
⋅
2
Π
⋅
--------------------------0----.-8----µ----------------------------
2
⋅
5----.-6----m----
2.7
⋅
1.2
k
+
2.4
m
=
2.0 k Ω
(R8)
CF
=
-----C-----o----⋅-----L2--
RF
=
-----6-----⋅---2---2----0---0----µ----⋅----1----2---µ----
2k
=
33 n F
(C2)
Further adjustments can be done on the work bench to fit the requirements and to compensate layout parasitic
components.
27/33
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