Controller Function
The controller logic block is the central processor that
determines the duty cycle of the high-side MOSFET
under different line, load, and temperature conditions.
Under normal operation, where the current-limit and
temperature protection are not triggered, the controller
logic block takes the output from the PWM comparator
and generates the driver signals for both high-side and
low-side MOSFETs. The break-before-make logic and
the timing for charging the bootstrap capacitors are
calculated by the controller logic block. The error signal
from the voltage-error amplifier is compared with the
ramp signal generated by the oscillator at the PWM
comparator and, thus, the required PWM signal is pro-
duced. The high-side switch is turned on at the begin-
ning of the oscillator cycle and turns off when the ramp
voltage exceeds the V
COMP
signal or the current-limit
threshold is exceeded. The low-side switch is then
turned on for the remainder of the oscillator cycle.
Current Limit
The internal, high-side MOSFET has a typical 7A peak
current-limit threshold. When current flowing out of LX
exceeds this limit, the high-side MOSFET turns off and
the synchronous rectifier turns on. The synchronous
rectifier remains on until the inductor current falls below
the low-side current limit. This lowers the duty cycle
and causes the output voltage to drop until the current
limit is no longer exceeded. The MAX15038 uses a hic-
cup mode to prevent overheating during short-circuit
output conditions.
During current limit, if V
FB
drops below 70% of V
REFIN
and stays below this level for 12μs or more, the
MAX15038 enters hiccup mode. The high-side
MOSFET and the synchronous rectifier are turned off
and both COMP and REFIN are internally pulled low. If
REFIN and SS are connected together, both are pulled
low. The part 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 part resumes
normal operation. Otherwise, the part reenters hiccup
mode again.
Soft-Start and REFIN
The MAX15038 utilizes an adjustable soft-start function
to limit inrush current during startup. An 8μA (typ) cur-
rent source charges an external capacitor connected to
SS. The soft-start time is adjusted by the value of the
external capacitor from SS to GND. The required
capacitance value is determined as:
where t
SS
is the required soft-start time in seconds. The
MAX15038 also features an external reference input
(REFIN). The IC regulates FB to the voltage applied to
REFIN. The internal soft-start is not available when
using an external reference. A method of soft-start
when using an external reference is shown in Figure 2.
Connect REFIN to SS to use the internal 0.6V reference.
Use a capacitor of 1nF minimum value at SS.
Undervoltage Lockout (UVLO)
The UVLO circuitry inhibits switching when V
DD
is below
2.55V (typ). Once V
DD
rises above 2.6V (typ), UVLO
clears and the soft-start function activates. A 50mV hys-
teresis 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.
Frequency Select (FREQ)
The switching frequency is resistor programmable from
500kHz to 2MHz. Set the switching frequency of the IC
with a resistor (R
FREQ
) connected from FREQ to GND.
R
FREQ
is calculated as:
where f
S
is the desired switching frequency in Hertz.
.
(.)
.
