MAX17000A Maxim Integrated Products, MAX17000A Datasheet - Page 24

• Manufacturer Part Number: MAX17000A
• Description: Complete DDR2 and DDR3 Memory Power-Management Solution
• Manufacturer: Maxim Integrated Products

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Complete DDR2 and DDR3 Memory
Power-Management Solution
•
•
The switching frequency and operating point (% ripple
current or LIR) determine the inductor value as follows:
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered
Figure 7a. Current-Sense Configurations (Sheet 1 of 2)
24
stresses and thus drives the selection of input
capacitors, MOSFETs, and other critical heat-con-
tributing components. Most notebook loads gener-
ally exhibit I
Switching Frequency: This choice determines the
basic trade-off between size and efficiency. The
optimal frequency is largely a function of maximum
input voltage, due to MOSFET switching losses that
are proportional to frequency and V
mum frequency is also a moving target, due to
rapid improvements in MOSFET technology that are
making higher frequencies more practical.
Inductor Operating Point: This choice provides
trade-offs between size vs. efficiency and transient
response vs. output noise. Low inductor values pro-
vide better transient response and smaller physical
size, but also result in lower efficiency and higher
output noise due to increased ripple current. The
minimum practical inductor value is one that causes
the circuit to operate at the edge of critical conduc-
tion (where the inductor current just touches zero
with every cycle at maximum load). Inductor values
lower than this grant no further size-reduction bene-
fit. The optimum operating point is usually found
between 20% and 50% ripple current.
______________________________________________________________________________________
L
=
⎛
⎜
⎝
A) OUTPUT SERIES RESISTOR SENSING
f
SW
LOAD
MAX17000A
×
I
V
LOAD MAX
IN
= I
-
PGND1
LOAD(MAX)
V
CSH
(
CSL
OUT
DH
DL
LX
)
×
Inductor Selection
LIR
N
N
H
L
x 80%.
⎞
⎟ ×
⎠
⎛
⎝ ⎝ ⎜
V
V
OUT
IN
IN
2
. The opti-
⎞
⎠ ⎟
C
D
IN
L
INPUT (V
L
IN
)
iron is inexpensive and can work well at 200kHz. The
core must be large enough not to saturate at the peak
inductor current (I
The minimum current-limit threshold must be high
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The val-
ley of the inductor current occurs at I
half the ripple current; therefore:
where I
threshold voltage divided by the output sense element
(inductor DCR or sense resistor).
The valley current limit is fixed at 17mV (min) across the
CSH to CSL differential input.
Special attention must be made to the tolerance and
thermal variation of the on-resistance in the case of DCR
sensing. Use the worst-case maximum value for R
from the inductor data sheet, and add some margin for
the rise in R
is to allow 0.5% additional resistance for each degree
Celsius of temperature rise, which must be included in
the design margin unless the design includes an NTC
thermistor in the DCR network to thermally compensate
the current-limit threshold.
The current-sense method (Figure 7) and magnitude
determine the achievable current-limit accuracy and
power loss. The sense resistor can be determined by:
R
L
SENSE RESISTOR
EQ
ESL
LIMIT(LOW)
I
LIMIT LOW
C
R
DCR
EQ
SENSE
I
PEAK
Setting the Valley Current Limit
(
R
with temperature. A good general rule
PEAK
SENSE
=
equals the minimum current-limit
I
)
LOAD MAX
>
):
I
LOAD MAX
= V
(
C
LIMIT
OUT
(
)
×
C
/I
EQ
⎛
⎝ ⎜
)
LIMIT
1
R
×
EQ
+
⎛
⎝ ⎜
=
LIR
1-
LOAD(MAX)
2
R
L
SENSE
LIR
ESL
⎞
⎠ ⎟
2
⎞
⎠ ⎟
minus
DCR