MAX1761 Maxim, MAX1761 Datasheet - Page 16

• Manufacturer Part Number: MAX1761
• Description: Small / Dual / High-Efficiency Buck Controller for Notebooks
• Manufacturer: Maxim

Small, Dual, High-Efficiency
Buck Controller for Notebooks
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
iron is inexpensive and works well at 250kHz. The core
must be large enough not to saturate at the peak induc-
tor current (I
The minimum current-limit threshold must be great
enough to support the maximum load current plus
some safety margin. For the circuit in Figure 1, with a
desired 2.5A maximum load current, the worst-case
current limit is set at 3.0A, providing a 20% safety mar-
gin. Under these conditions, the valley of the inductor
current waveform occurs at:
The required valley current is I
✕
age must be scaled taking into account the tolerance of
the CS_ current-limit threshold and the maximum MOS-
FET drain-source on-resistance (R
over temperature. The minimum current-limit threshold
at the CS_ pins is 92mV. The worst-case maximum
value for (R
2.56A, the voltage developed across the low-side
switch is 128mV. A resistive voltage-divider with a
0.703 attenuation ratio is necessary to scale this volt-
age to the 92mV CS_ threshold.
A current-sense resistor can be used if a more accu-
rate current limit is needed than is available when using
the MOSFET (R
resistor between the source of the low-side MOSFET
and ground provides a very accurate sense point for
the CS_ inputs. Alternatively, a small sense resistor can
be used in series with the low-side MOSFET to ballast
the device and reduce the temperature coefficient of
the current limit when sensing at the inductor’s
switched node. This provides a compromise between
sensing across the MOSFET device alone or using a
large sense resistor.
16
2.5A = 2.56A. Next, the current-sense feedback volt-
I
______________________________________________________________________________________
VALLEY
I
PEAK
L
=
= I
20
PEAK
= I
DS(ON)
LOAD(MAX)
V
LOAD(MAX)
(1 - 0.5 LIR) I
(1 - 0.5 LIR) I
×
DS(ON)
):
2 5 20
350
.
V
) over temperature is 50mΩ. At
kHz
(
(Figure 6). Placing the sense
V
×
- 1/2 LIR
-
- 1/2 LIR
0 35 2 5
2 5
Setting Current Limit
.
LOAD(MAX)
LOAD(MAX)
.
V
VALLEY
×
)
✕
.
✕
A
I
LOAD(MAX)
DS(ON)
I
LOAD(MAX)
= 3A - 1/2 (0.35)
=
7 1
.
µ
H
) variation
=
=
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 the stability criterion.
In CPU V
the output is subject to violent load transients, the out-
put capacitor’s size 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 non-CPU applications, the output capacitor’s size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple:
The actual required µF capacitance value relates to the
physical size needed to achieve low ESR as well as to
the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value (this is true of tanta-
lums, SP, POS, and other electrolytics).
When using low-capacitance filter capacitors, such as
ceramic or polymer types, capacitor size is usually
determined by the capacitance needed to prevent
V
transients. Generally, once enough capacitance is
added to meet the V
the rising load edge is no longer a problem (see the
V
overshoot due to stored inductor energy can be calcu-
lated as:
where I
Stability is determined by the value of the ESR zero rel-
ative to the switching frequency. The point of instability
is given by the following equation:
SAG
SAG
and V
equation in Design Procedure). The amount of
PEAK
CORE
SOAR
is the peak inductor current.
converters and other applications where
R
ESR
R
from causing problems during load
ESR
∆V
Output Capacitor Selection
≤
SOAR
ƒ
≈
LIR I
ESR
≤
Stability Considerations
(
L
I
LOAD(MAX)
2CV
×
×
requirement, undershoot at
=
I
PEAK
V
Vp - p
LOAD(MAX)
ƒ
π
DIP
OUT
2
)