Rainbow Electronics MAX1855 User Manual
Page 22
MAX1716/MAX1854/MAX1855
High-Speed, Adjustable, Synchronous Step-Down
Controllers with Integrated Voltage Positioning
22
______________________________________________________________________________________
mal frequency is largely a function of maximum input
voltage, due to MOSFET switching losses that are pro-
portional to frequency and V+
2
. The optimum frequency
is also a moving target, due to rapid improvements in
MOSFET technology that are making higher frequen-
cies more practical.
Inductor operating point: This choice provides trade-
offs between size vs. efficiency. Low inductor values
cause large ripple currents, resulting in the smallest
size, but poor efficiency and high output noise. The
minimum practical inductor value is one that causes the
circuit to operate at the edge of critical conduction
(where the inductor current just touches zero with every
cycle at maximum load). Inductor values lower than this
grant no further size-reduction benefit.
The MAX1716/MAX1854/MAX1855’s pulse-skipping
algorithm initiates skip mode at the critical-conduction
point. Thus, the inductor operating point also deter-
mines the load-current value at which PFM/PWM
switchover occurs. The optimum point is usually found
between 20% and 50% ripple current.
The inductor ripple current impacts transient-response
performance, especially at low V
IN
- V
OUT
differentials.
Low inductor values allow the inductor current to slew
faster, replenishing charge removed from the output fil-
ter capacitors by a sudden load step. The amount of
output sag is also a function of the maximum duty fac-
tor, which can be calculated from the on-time and mini-
mum 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)
= 18A, V
IN
= 7V, V
OUT
= 1.6V,
f
SW
= 300kHz, 30% ripple current or LIR = 0.3.
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 (IPEAK).
I
PEAK
= I
LOAD(MAX)
+ (I
LOAD(MAX)
× LIR / 2)
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 val-
ley of the inductor current occurs at I
LOAD(MAX)
minus
half of the ripple current; therefore:
I
LIMIT(LOW)
> I
LOAD(MAX)
- (I
LOAD(MAX)
× LIR / 2)
where I
LIMIT(LOW)
equals the minimum current-limit
threshold voltage divided by R
SENSE
. For the 120mV
default setting, the minimum current-limit threshold is
110mV.
Connect ILIM to V
CC
for a default 120mV current-limit
threshold. In the adjustable mode, the current-limit
threshold is precisely 1/10th the voltage seen at ILIM.
For an adjustable threshold, connect a resistive divider
from REF to GND, with ILIM connected to the center
tap. The external 0.5V to 2.0V adjustment range corre-
sponds to a current-limit threshold of 50mV to 200mV.
When adjusting the current limit, use 1% tolerance
resistors and a 10µA divider current to prevent a signifi-
cant increase of errors in the current-limit value.
Output Capacitor Selection
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
CORE
converters and other applications where
the output is subject to violent load transients, the out-
put 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(MAX)
/ I
LOAD(MAX)
The actual µF 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 voltage rating
rather than by capacitance value (this is true of tanta-
lums, OS-CONs, and other electrolytics).
L
V
V
V
V
kHz
A
H
=
Ч
−
Ч
Ч
Ч
=
1 6
7
1 6
7
300
0 30
18
0 76
.
(
.
)
.
.
µ
L
V
V
V
V
LIR
I
OUT
OUT
SW
LOAD MAX
=
Ч + −
(
)
+ Ч
Ч
Ч
ƒ
(
)
V
I
I
L
K
V
V
t
C
V
K
V
V
V
t
SAG
LOAD
LOAD
OUT
OFF MIN
OUT
OUT
OUT
OFF MIN
=
−
Ч Ч
Ч
+
−
Ч
Ч
Ч
Ч
+ −
+
−
(
)
(
)
(
)
1
2
2
2