MAX17000A Maxim Integrated Products, MAX17000A Datasheet - Page 25

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

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Figure 7b. Current-Sense Configurations (Sheet 2 of 2)
For the best current-sense accuracy and overcurrent
protection, use a 1% tolerance current-sense resistor
between the inductor and output as shown in Figure 7a.
This configuration constantly monitors the inductor cur-
rent, allowing accurate current-limit protection.
However, the parasitic inductance of the current-sense
resistor can cause current-limit inaccuracies, especially
when using low-value inductors and current-sense
resistors. This parasitic inductance (L
celled by adding an RC circuit across the sense resis-
tor with an equivalent time constant:
Alternatively, low-cost applications that do not require
highly accurate current-limit protection could reduce
the overall power dissipation by connecting a series RC
circuit across the inductor (Figure 7b) with an equiva-
lent time constant:
and:
where R
and R
the worst-case inductance and R
by the inductor manufacturer, adding some margin for
the inductance drop over temperature and load.
DCR
CS
is the inductor’s series DC resistance. Use
is the required current-sense resistance,
B) LOSSLESS INDUCTOR SENSING
MAX17000A
R
C
R
DCR
______________________________________________________________________________________
EQ
CS
×
=
=
PGND1
R
R
C
CSH
CSL
EQ
1
L
EQ
DH
DL
LX
R
+
2
R
=
×
2
R
⎡
⎢
⎣
L
×
SENSE
R
Complete DDR2 and DDR3 Memory
1
ESL
1
R
N
N
H
L
DCR
DCR
+
R
1
2
ESL
⎤
⎥
⎦
values provided
) can be can-
C
Power-Management Solution
D
IN
L
INPUT (V
IN
R
)
1
L
INDUCTOR
The DH and DL drivers are optimized for driving moder-
ate-sized high-side, and larger low-side power MOSFETs.
This is consistent with the low duty factor seen in note-
book applications, where a large V
exists. The high-side gate driver (DH) sources and sinks
1.2A, and the low-side gate driver (DL) sources 1.0A and
sinks 2.4A. This ensures robust gate drive for high-cur-
rent applications. The DH floating high-side MOSFET dri-
ver is powered by an internal boost switch charge pump
at BST, while the DL synchronous-rectifier driver is pow-
ered directly by the 5V bias supply (V
The output filter capacitor must have low enough effec-
tive series resistance (ESR) to meet output ripple and
load-transient requirements, yet have high enough ESR
to satisfy stability requirements.
In core and chipset converters and other applications
where the output is subject to large-load transients, the
output capacitor’s size typically depends on how much
ESR is needed to prevent the output from dipping too
low under a load transient. Ignoring the sag due to finite
capacitance:
In low-power applications, the output capacitor’s size
often depends on how much ESR is needed to maintain
an acceptable level of output ripple voltage. The output
ripple voltage of a step-down controller equals the total
inductor ripple current multiplied by the output capaci-
tor’s ESR.
R
DCR
R
C
2
EQ
PWM Output Capacitor Selection
(
R
FOR THERMAL COMPENSATION:
R2 SHOULD CONSIST OF AN NTC RESISTOR IN
SERIES WITH A STANDARD THIN-FILM RESISTOR.
ESR
MOSFET Gate Drivers (DH, DL)
+
C
R
OUT
PCB
R
R
CS
DCR
)
≤
=
=
R1 + R2
Δ
C
R2
L
EQ
I
LOAD MAX
V
[
STEP
R
R1 R2
IN
DCR
1
DD
+
(
- V
1
).
]
OUT
)
differential
25