Rainbow Electronics MAX1717 User Manual
Page 25
MAX1717
Dynamically Adjustable, Synchronous
Step-Down Controller for Notebook CPUs
______________________________________________________________________________________
25
The amount of output sag is also a function of the
maximum duty factor, which can be calculated from
the on-time and minimum off-time:
where t
OFF(MIN)
is the minimum off-time (see Electrical
Characteristics) and K is from Table 3.
Inductor Selection
The switching frequency and operating point (% ripple or
LIR) determine the inductor value as follows:
Example: I
LOAD(MAX)
= 14A, V
IN
= 7V, V
OUT
= 1.6V,
f
SW
= 300kHz, 30% ripple current or LIR = 0.30.
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
PEAK
).
I
PEAK
= I
LOAD(MAX)
+ (LIR / 2) I
LOAD(MAX)
Setting the Current Limit
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 valley
of the inductor current occurs at I
LOAD(MAX)
minus half
of the ripple current; therefore:
I
LIMIT(LOW)
> I
LOAD(MAX)
- (LIR / 2) I
LOAD(MAX)
where I
LIMIT(LOW)
equals the minimum current-limit
threshold voltage divided by the R
DS(ON)
of Q2. For the
MAX1717, the minimum current-limit threshold (100mV
default setting) is 90mV. Use the worst-case maximum
value for R
DS(ON)
from the MOSFET Q2 data sheet, and
add some margin for the rise in R
DS(ON)
with tempera-
ture. A good general rule is to allow 0.5% additional
resistance for each °C of temperature rise.
Examining the Figure 1 example with a Q
2
maximum
R
DS(ON)
= 5.5m
Ω at T
J
= +25°C and 7.5m
Ω at T
J
=
+100°C reveals the following:
I
LIMIT(LOW)
= 90mV / 7.5m
Ω = 11.9A
and the required valley current limit is:
I
LIMIT(LOW)
> 14A - (0.3012) 14A = 11.9A
Therefore, the circuit can deliver the full-rated 14A
using the default ILIM threshold.
When delivering 14A of output current, the worst-case
power dissipation of Q2 is 1.48W. With a thermal resis-
tance of 60°C/W and each MOSFET dissipating 0.74W,
the temperature rise of the MOSFETs is 60°C/W x
0.74W = 44.5°C, and the maximum ambient tempera-
ture is +100°C - 44.5°C = +55.5°C. To operate at a
higher ambient temperature, choose lower R
DS(ON)
MOSFETs or reduce the thermal resistance. You could
also raise the current-limit threshold, allowing operation
with a higher MOSFET junction temperature.
Connect ILIM to V
CC
for a default 100mV current-limit
threshold. For an adjustable threshold, connect a resistor
divider from REF to GND, with ILIM connected to the cen-
ter tap. The external adjustment range of 0.5V to 3V cor-
responds to a current-limit threshold of 50mV to 300mV.
When adjusting the current limit, use 1% tolerance resis-
tors and a 10µA divider current to prevent a significant
increase of errors in the current-limit tolerance.
Output Capacitor Selection
The output filter capacitor must have low enough effective
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 without tripping
the OVP circuit.
In CPU V
CORE
converters and other applications where
the output is subject to violent 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:
R
ESR
≤ V
STEP
/ I
LOAD(MAX)
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.
L
V V
V
V
kHz
A
H
=
Ч
Ч
Ч
=
−
1 6
7
1 6
7
300
0 30
14
0 98
.
(
.
)
.
.
µ
L
V
V
V
V
LIR
I
OUT
IN
OUT
IN
SW
LOAD MAX
f
=
(
)
Ч
Ч
Ч
−
(
)
V
I
I
L K
V
V
t
C
V
K
V
V
V
t
SAG
LOAD
LOAD
OUT
IN
OFF MIN
OUT
OUT
IN
OUT
IN
OFF MIN
=
Ч
+
Ч
Ч
−
−
−
(
)
(
)
(
)
1
2
2
2