Rainbow Electronics MAX15040 - Manual (Page 10)

MAX15040
High-Efficiency, 4A, Step-Down Regulator with
Integrated Switches in 2mm x 2mm Package
10 ______________________________________________________________________________________
Current Limit
The internal, high-side MOSFET has a typical 7A peak cur-
rent-limit threshold. When current flowing out of LX
exceeds this limit, the high-side MOSFET turns off and the
low-side MOSFET turns on. The low-side MOSFET
remains on until the inductor current falls below the low-
side current limit. This lowers the duty cycle and causes
the output voltage to droop until the current limit is no
longer exceeded. The MAX15040 uses a hiccup mode to
prevent overheating during short-circuit output conditions.
During current limit, if V FB drops below 70% of
V REFIN/SS and stays below this level for typically 36µs
(12µs min) or more, the device enters hiccup mode.
The high-side MOSFET and the low-side MOSFET turn
off and both COMP and REFIN/SS are internally pulled
low. The device remains in this state for 896 clock
cycles and then attempts to restart for 112 clock
cycles. If the fault-causing current limit has cleared, the
device resumes normal operation. Otherwise, the
device reenters hiccup mode.
Soft-Start and Reference Input (REFIN/SS)
The MAX15040 utilizes an adjustable soft-start function
to limit inrush current during startup. An 8µA (typ) cur-
rent source charges an external capacitor connected to
REFIN/SS. The soft-start time is adjusted by the value of
the external capacitor from REFIN/SS to GND. The
required capacitance value is determined as:
where t SS is the required soft-start time in seconds.
Connect a minimum 1nF capacitor between REFIN/SS
and GND. REFIN/SS is also an external reference input
(REFIN/SS). The device regulates FB to the voltage
applied to REFIN/SS. The internal soft-start is not avail-
able when using an external reference. Figure 2 shows
a method of soft-start when using an external refer-
ence. If an external reference is not applied, the device
uses the internal 0.6V reference.
Undervoltage Lockout (UVLO)
The UVLO circuitry inhibits switching when V DD is
below 1.9V (typ). Once V DD rises above 2V (typ), UVLO
clears and the soft-start function activates. A 100mV
hysteresis is built in for glitch immunity.
BST
The gate-drive voltage for the high-side, n-channel
switch is generated by a flying-capacitor boost circuit.
The capacitor between BST and LX is charged from the
V
IN supply while the low-side MOSFET is on. When the
low-side MOSFET is switched off, the voltage of the
capacitor is stacked above LX to provide the necessary
turn-on voltage for the high-side internal MOSFET.
Power-Good Output (PWRGD)
PWRGD is an open-drain output that goes high
impedance when V
FB is above 92.5% x V REFIN/SS and
V REFIN/SS is above 0.54V. PWRGD pulls low when V FB
is below 90% of V REFIN/SS for at least 48 clock cycles
or V REFIN/SS is below 0.54V. PWRGD is low during
shutdown.
Setting the Output Voltage
The MAX15040 output voltage is adjustable from 0.6V
to 90% of V
IN by connecting FB to the center tap of a
resistor-divider between the output and GND (Figure
3). To determine the values of the resistor-divider, first
select the value of R3 between 2k
Ω and 10kΩ. Then
use the following equation to calculate R4:
R4 = (V FB x R3)/(V OUT - V FB )
where V FB is equal to the reference voltage at
REFIN/SS and V OUT is the output voltage. For V OUT =
0.6V, remove R4. If no external reference is applied at
REFIN/SS, the internal reference is automatically select-
ed and V
FB becomes 0.6V.
C
A t
V
SS =
× 8
0 6
µ
.
C R2
R1
REFIN/SS
MAX15040
Figure 2. Typical Soft-Start Implementation with External
Reference
LX
FB
R3
R4
MAX15040
Figure 3. Setting the Output Voltage with a Resistor Voltage-
Divider