Rainbow Electronics MAX1813 User Manual
Page 25
MAX1813
Dynamically-Adjustable, Synchronous Step-Down
Controller with Integrated Voltage Positioning
______________________________________________________________________________________
25
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)
= 22A, V
IN
= 12V, V
OUT
= 1.4V,
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 (I
PEAK
).
I
PEAK
= I
LOAD(MAX)
+ (I
LOAD(MAX)
x 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:
where I
LIMIT(LOW)
equals the minimum current-limit
threshold voltage divided by R
SENSE
. For the 50mV
default setting, the minimum current-limit threshold is
40mV.
Connect ILIM to V
CC
for a default 50mV 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 50mV to 200mV current-limit threshold.
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 tolerance.
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 large 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(MAX)
/ I
LOAD(MAX)
The actual 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).
When using low-capacity filter capacitors such as
ceramic or polymer types, capacitor size is usually
determined by the capacity needed to prevent V
SAG
and V
SOAR
from causing problems during load tran-
sients. Generally, once enough capacitance is added
to meet the overshoot requirement, undershoot at the
rising load edge is no longer a problem (see the V
SAG
equation in the Design Procedure). The amount of over-
shoot due to stored inductor energy can be calculated
as:
where I
PEAK
is the peak inductor current.
Output Capacitor Stability Considerations
Stability is determined by the value of the ESR zero rel-
ative to the switching frequency. The boundary of insta-
bility is given by the following equation:
V
I
L
2C
V
SOAR
PEAK
2
OUT OUT
≈
I
) >I
-
I
LIR
2
LIMIT(LOW
LOAD(MAX)
LOAD(MAX)
L
1.4V (12V -1.4V)
12V 300kHz 22A 0.3
0.62 H
=
Ч
Ч
Ч
Ч
=
µ
L
V
(V
V
)
V
I
LIR
OUT
IN
OUT
IN
SW
LOAD(MAX)
=
Ч
−
Ч ƒ
Ч
Ч
V
L(I
-I
)
V
K
V
+ t
2C
V
(V
- V
)K
V
- t
SAG
LOAD1 LOAD2
2
OUT
IN
OFF(MIN)
OUT OUT
IN
OUT
IN
OFF(MIN)
=