MAX8717ETI+ Maxim Integrated Products, MAX8717ETI+ Datasheet - Page 22

• Manufacturer Part Number: MAX8717ETI+
• Description: IC CNTRLR PWR SUP 28-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8717ETI+

• Applications: Controller, Notebook Computers
• Voltage - Input: 4 ~ 26 V
• Number Of Outputs: 2
• Voltage - Output: 3.3V, 5V, 1 ~ 5.5 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN Exposed Pad
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Interleaved High-Efficiency, Dual Power-Supply
Controllers for Notebook Computers
look for nonstandard values, which can provide a better
compromise in LIR across the input voltage range. If
using a swinging inductor (where the no-load induc-
tance decreases linearly with increasing current), evalu-
ate the LIR with properly scaled inductance values. For
the selected inductance value, the actual peak-to-peak
inductor ripple current (ΔI
Ferrite cores are often the best choice, although pow-
dered 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 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 total output voltage sag is the sum of the voltage
sag while the inductor is ramping up and the voltage
sag before the next pulse can occur:
where D
Characteristics ), T is the switching period (1 / f
ΔT equals V
x I
overshoot during a full-load to no-load transient due to
stored inductor energy can be calculated as:
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
peak inductor current occurs at I
the ripple current; therefore:
22
V
MAX
SAG
______________________________________________________________________________________
/ (V
=
MAX
2
C
IN
I
PEAK
OUT IN
OUT
ΔI
- V
is maximum duty factor (see the Electrical
L I
INDUCTOR
(
V
Δ
(
OUT
V
SOAR
/ V
=
LOAD MAX
I
IN
LOAD MAX
×
) when in skip mode. The amount of
D
x T when in PWM mode, or L x 0.2
(
MAX
≈
Setting the Current Limit
PEAK
(
(
=
Δ
INDUCTOR
)
I
2
−
V
LOAD MAX
)
C
2
OUT IN
V
):
OUT
OUT OUT
)
Transient Response
V
+
IN OSC
(
(
Δ
V
)
V
ƒ
I
+
INDUCTOR
LOAD(MAX)
) is defined by:
Δ
−
)
I
LOAD MAX
)
2
V
2
L
OUT
L
C
(
IN
)
OUT
- V
OSC
)
plus half
(
T
OUT
−
), and
Δ
T
dif-
)
where I
old voltage divided by the current-sense resistance
(R
current-limit threshold is 50mV.
Connect ILIM_ to V
threshold. In adjustable mode, the current-limit thresh-
old is precisely 1/10 the voltage seen at ILIM_. For an
adjustable threshold, connect a resistive divider from
REF to analog ground (GND) with ILIM_ connected to
the center tap. The external 500mV to 2V adjustment
range corresponds to a 50mV to 200mV current-limit
threshold. When adjusting the current limit, use 1% tol-
erance resistors and a divider current of approximately
10µA to prevent significant inaccuracy in the current-
limit tolerance.
The current-sense method (Figure 8) and magnitude
determines the achievable current-limit accuracy and
power loss. Typically, higher current-sense limits pro-
vide tighter accuracy, but also dissipate more power.
Most applications employ a current-limit threshold
(V
determined by:
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 8a.
This configuration constantly monitors the inductor cur-
rent, allowing accurate current-limit protection.
Alternatively, high-power applications that do not
require highly accurate current-limit protection may
reduce the overall power dissipation by connecting a
series RC circuit across the inductor (Figure 8b) with an
equivalent time constant:
where R
configuration, the current-sense resistance equals the
inductor’s DC resistance (R
case inductance and R
tor manufacturer, adding some margin for the
inductance drop over temperature and load.
LIM
SENSE
) of 50mV to 100mV, so the sense resistor can be
LIMIT_
L
). For the 50mV default setting, the minimum
I
is the inductor’s series DC resistance. In this
LIMIT
equals the minimum current-limit thresh-
R
>
SENSE_
I
LOAD MAX
CC
R
L
L
(
L
for a default 50mV current-limit
=
= V
values provided by the induc-
C
EQ
SENSE
)
LIM_
+
×
⎛
⎜
⎝
R
Δ
/ I
EQ
I
INDUCTOR
= R
LIM_
2
L
). Use the worst-
⎞
⎟
⎠