Datasheet

MAX8543/MAX8544
Step-Down Controllers with Prebias Startup,
Lossless Sensing, Synchronization, and OVP
20 ______________________________________________________________________________________
2) If the resulting value of R
ILIM
is negative, either
increase P
FB
or choose a low-side MOSFET with a
lower R
DS(ON)
. The latter is preferred as it increas-
es the efficiency and results in a lower short-circuit
current.
To set the constant current limit for the latch-up mode,
only R
ILIM
is used. The equation for R
ILIM
below sets
the current-limit threshold at 1.2 times the maximum-
rated output current:
Similarly, I
VALLEY
is the value of the inductor valley
current at maximum load, R
DS(ON)
is the maximum on-
resistance of the low-side MOSFET at the highest oper-
ating junction temperature.
Peak Current Limit
Peak inductor current-limit threshold (V
TH
) has four
possible settings through ILIM (MAX8543) or ILIM1
(MAX8544) as shown in Table 3 below. The resulting
current limit is calculated as:
where R
DC
is either the DC resistance of the inductor or
the value of the optional current-sense resistor.
Note that V
ILIM
is a logic-level setting, and can allow a
variation of ±0.1 x V
VL
without affecting V
TH
. To ensure
maximum output current, use the minimum value of V
TH
from each setting, and the maximum R
DC
values at the
highest expected operating temperature. The DC resis-
tance of the inductor’s copper wire has a +0.22%/°C
temperature coefficient.
To use the DC resistance of the output inductor for cur-
rent sensing, an RC circuit is added (see Figure 7). The
RC time constant is set to be twice the inductor (L / R
DC
)
time constant. Pick the value of R4 in the range of 470Ω
to 2kΩ, and then calculate the capacitor value from: C9
= 2L / (R
DC
× R4). Add a resistor (R5) equal in value to
R4 to the CS- connection to minimize input-offset error.
The equivalent current-sense resistance is equal to the
DC resistance of the inductor (R
DC
).
To use a current-sense resistor, connect the resistor as
shown in Figure 8. Since most current-sense resistors
have inductance, the RC circuit is also required
and is calculated in the same manner as inductor
current sensing. Place C11 close to CS+ and CS- pins
to decouple the high-frequency noise pickup. Place
C10 (same value as C9) across R5 to aid in short-
circuit recovery.
I
V
R
I
LIM
TH
DC
PP
=−
−
2
R
IR
A
ILIM
VALLEY DS ON
=
××12
1
.
()
μ
Table 3. ILIM Current-Limit Threshold
Settings
V
ILIM
RECOMMENDED
ILIM CONNECTION
V
TH
MIN
(mV)
V
TH
TYP
(mV)
V
TH
MAX
(mV)
0 GND
38.5 50
56.5
1/3 V
VL
Voltage-divider:
100kΩ from ILIM/ILIM1 to GND
200kΩ from ILIM/ILIM1 to VL
85.0 100
115.0
2/3 V
VL
Voltage-divider:
200kΩ from ILIM/ILIM1 to GND
100kΩ from ILIM/ILIM1 to VL
127.5 150
172.5
V
VL
VL
170.0 200
230.0
MAX8543/
MAX8544
C9
V
OUT
R4
R5
R3
L1
LX
CS-
CS+
C11
C10
Figure 8. Using a Current-Sense Resistor
MAX8543/
MAX8544
C9
V
OUT
R4
R5
L1
LX
CS-
CS+
C11
C10
Figure 7. Inductor R
DC
Current Sensing