MAX1715 Maxim, MAX1715 Datasheet - Page 18

• Manufacturer Part Number: MAX1715
• Description: Ultra-High Efficiency / Dual Step-Down Controller for Notebook Computers
• Manufacturer: Maxim

Ultra-High Efficiency, Dual Step-Down
Controller for Notebook Computers
The inductor ripple current also impacts transient-
response performance, especially at low V
ferentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The amount of output sag is also a function of the maxi-
mum duty factor, which can be calculated from the on-
time and minimum off-time:
where
where minimum off-time = 400ns typ (see Table 5).
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as fol-
lows:
Example: I
300kHz, 35% ripple current or LIR = 0.35:
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 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 great
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 of the ripple current; therefore:
where I
voltage divided by the R
MAX1715, the minimum current-limit threshold (100mV
default setting) is 90mV. Use the worst-case maximum
value for R
add some margin for the rise in R
18
DUTY
I
______________________________________________________________________________________
LIMIT(LOW)
I
PEAK
V
LIMIT(LOW)
SAG
=
L
LOAD(MAX)
DS(ON)
= I
K (V
=
L =
7 300kHz 0.33 8A
=
LOAD(MAX)
⋅
OUT
> I
2
Determining the Current Limit
V
1.6V (7 - 1 6)
PEAK
from the MOSFET Q2 data sheet, and
⋅
IN
LOAD(MAX)
C
= minimum current-limit threshold
V
K (V
⋅ ⋅
F
+ 0.075V) V
OUT
= 8A, V
(
f LIR I
⋅
):
∆
DUTY V
I
OUT
LOAD MAX
⋅
+ [(LIR / 2) · I
(V
IN
⋅
⋅
IN
+ 0.075V) V
DS(ON)
- (LIR / 2) I
(
LOAD(MAX)
(
- V
⋅
Inductor Selection
IN MIN
= 7V, V
OUT
OUT
(
DS(ON)
)
)
=
2
)
)
+ min off - time
LOAD(MAX)
1.6 H
⋅
of Q2. For the
LOAD(MAX)
-
L
OUT
V
LOAD(MAX)
OUT
µ
IN
with tempera-
IN
= 1.6V, f =
- V
)
OUT
]
minus
dif-
ture. A good general rule is to allow 0.5% additional
resistance for each °C of temperature rise.
Examining the 8A circuit example with a maximum
R
lowing:
7.5A is greater than the valley current of 6.6A, so the
circuit can easily deliver the full-rated 8A using the
default 100mV nominal ILIM threshold.
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. Also, the capacitance
value must be high enough to absorb the inductor
energy going from a full-load to no-load condition with-
out tripping the overvoltage protection circuit.
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 microfarad capacitance value required
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 volt-
age rating rather than by capacitance value (this is true
of tantalums, OS-CONs, and other electrolytics).
When using low-capacity filter capacitors such as
ceramic or polymer types, capacitor size is usually
determined by the capacity needed to prevent VSAG
and VSOAR from causing problems during load tran-
sients. Also, the capacitance must be great enough to
prevent the inductor’s stored energy from launching the
output above the overvoltage protection threshold.
Generally, once enough capacitance is added to meet
the overshoot requirement, undershoot at the rising
load edge is no longer a problem (see the VSAG equa-
tion in the Design Procedure).
DS(ON)
CORE
= 12mΩ at high temperature reveals the fol-
I
LIMIT(LOW)
converters and other applications where
R
ESR
R
ESR
Output Capacitor Selection
≤
= 90mV / 12mΩ = 7.5A
LIR I
≤
I
LOAD MAX
⋅
LOAD MAX
Vp p
V
DIP
(
-
(
)
)