SC1480
POWER MANAGEMENT
Applications Information (Cont.)
The selection of the ripple current is based on the maxi- RDSON of the MOSFET is a nominal 0.013 Ohms, ac-
mum output current and tends to be between 20% to counting for increased temperature effects use 0.015
50% of the maximum load current. Again, cost, size and Ohm.
efficiency all play a part in the selection process.
0.015• 6.5
RILIM =
= 10k
10µA
Design Example
The inductor chosen was a Panasonic 4µH, 11A induc-
The following design example is for the evaluation board
schematic shown on page 16. While most DDR supplies
have a maximum load of 3A, the following design was
meant for applications beyond DDR memory. Therefore,
this design will have an input voltage from 5V to 19V,
with an output voltage of 1.25V at 5A of load current.
tor.
Similarly, using a Sense resistor to obtain a more accu-
rate current limit would make use of the valley current
equation. Thus, for a 0.015 Ohm resistor the RILIM would
calculate to the same 10k Ohm ILIM resistor will be
effecting the efficiency budget.
6 • 6 • 0.015 = 0.54W
Inductor Selection
Output Capacitor Selection
The switching frequency is set to 300kHz which yields a
good trade-off of size and efficiency. RTON is chosen to
be 1M Ohm for a switching frequency of 300kHz. Be-
cause ripple voltage is used as the feedback mechanism
of this device, this leads to the choice of the ripple cur-
rent being 10% of load current. This will give a nice ripple
voltage waveform for ensuring proper PWM triggering for
this type of controller.
The output filter capacitor must have low effective se-
ries resistance (ESR) to meet the output ripple and load
transient requirements, at the same time have high
enough ESR to satisfy stability requirements. In addition,
the value of output capacitance must be high enough to
absorb the inductor energy going from full-load to no-
load without tripping the overvoltage protection circuit.
For CPU load transients, how much ESR is needed de-
pends upon output voltage variation limits under a CPU
load transient. The ESR for this condition is given:
∆IL=0.1•5=0.5A
VOUT • (V − VOUT )• T
IN
L =
V • ∆IL
IN
∆VOUT
ESR =
L =4µH
I
LOAD(MAX)
Setting the Current Limit
In non CPU applications, the output capacitor size de-
pends on how much ESR is needed to maintain an ac-
ceptable level of output voltage ripple. Under these con-
ditions the ESR value is given:
The minimum current-limit threshold must be high enough
to support the maximum load current. The valley of the
inductor current occurs at:
∆IL
∆VOUT(p -p)
ESR =
ILoad(Max) −
, (see Figure 2) therefore:
2
I
LOAD(MAX)
∆IL
ILIM(Low) >ILoad(Max) -
However, for most CPU applications the minimum capaci-
tance required is limited by the energy absorption of the
output capacitor. The equation for determining the mini-
mum capacitance can be found by the following equa-
tion:
2
The inductor must not saturate under all conditions of
operation. If the current limit is set to 6.5A the maximum
current through the inductor will be:
6.5+∆IL=7A
2
Setting the over current to 6.5A is calculated as follows:
L
OUT•IOUT
CMIN
=
RDSON•ILOC
RILIM=
V
F
2 −V2
I
10µA
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