Rainbow Electronics MAX1541 User Manual
Page 27
(Figures 3 and 5). The actual peak current is greater
than the valley current-limit threshold by an amount
equal to the inductor ripple current. Therefore, the exact
current-limit characteristic and maximum load capability
are a function of the current-sense resistance, inductor
value, and battery voltage. When combined with the
undervoltage protection circuit, this current-limit method
is effective in almost every circumstance. Figure 6
shows the valley current-limit threshold point.
In forced-PWM mode, the MAX1540/MAX1541 also
implement a negative current limit to prevent excessive
reverse inductor currents when V
OUT
is sinking current.
The negative current-limit threshold is set to approxi-
mately 120% of the positive current limit and tracks the
positive current limit when ILIM is adjusted.
The current-limit threshold is adjusted with an external
resistor-divider at ILIM_. A 2µA to 20µA divider current
is recommended for accuracy and noise immunity. The
current-limit threshold adjustment range is from 25mV
to 200mV. In the adjustable mode, the current-limit
threshold voltage is precisely 1/10th the voltage seen at
ILIM_. The threshold defaults to 50mV when ILIM_ is
connected to V
CC
. The logic threshold for switchover to
the 50mV default value is approximately V
CC
- 1V.
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors do not corrupt the dif-
ferential current-sense signals seen by CSP_ and
CSN_. Place the IC close to the sense resistor with
short, direct traces, making a Kelvin-sense connection
to the current-sense resistor.
MAX1540/MAX1541
Dual Step-Down Controllers with Saturation
Protection, Dynamic Output, and Linear Regulator
______________________________________________________________________________________
27
Figure 5. MAX1540/MAX1541 Current-Limit Functional Diagram
S
R
Q
QUAD-LEVEL
DECODE
SATURATION
LIMIT
VALLEY CURRENT
LIMIT
ZERO
CROSSING
DH DRIVER
9R
R
0.5V
CSP_
SKIP
CSN_
ILIM_
6
µA
V
CC
- 1.0V
LSAT
Figure 6. Valley Current-Limit Threshold Point
INDUCTOR CURRENT
I
LIMIT
I
LOAD
0
TIME
I
PEAK
I
LIM(VAL)
= I
LOAD(MAX)
1-
LIR
2
(
)