MAX8505
Current Limit
The MAX8505 offers both high-side and low-side
current limits. The high-side current limit monitors the
inductor peak current and the low-side current limit
monitors the inductor valley current. Current-limit thresh-
olds are 6A (typ) for high side and 3.8A (typ) for low
side. If the output inductor current exceeds the high-
side current limit during its on-time, the high-side MOS-
FET turns off and the synchronous rectifier turns on. The
inductor current is continuously monitored during the
on-time of the low-side MOSFET. If the inductor current
is still above the low-side current limit at the moment of
the next clock cycle, the high-side MOSFET is not
turned on and the low-side MOSFET is kept on to contin-
ue discharging the output inductor current. Once the
inductor current is below the low-side current limit, the
high-side MOSFET is turned on at the next clock cycle.
If the inductor current stays less than the high-side cur-
rent limit during the minimum on duty ratio, the normal
operation resumes at the next clock cycle. Otherwise,
the current-limit operation continues.
V
CC
Decoupling
Due to the high switching frequency and tight output
tolerance (1%), decouple V
CC
from IN with a 10Ω
resistor and bypass to GND with a 0.1µF capacitor.
Place the capacitor as close to V
CC
as possible.
Bootstrap (BST)
Gate-drive voltage for the high-side N-channel switch is
generated by a bootstrapped capacitor boost circuit.
The bootstrapped capacitor is connected between the
BST pin and LX. When the low-side N-channel MOSFET
is on, it forces LX to ground and charges the capacitor
to V
IN
through diode D1. When the low-side N-channel
MOSFET turns off and the high-side N-channel MOSFET
turns on, LX is pulled to V
IN
. D1 prevents the capacitor
from discharging into V
IN
and the voltage on the boot-
strapped capacitor is boosted above V
IN
. This provides
the necessary voltage for the high driver. A Schottky
diode should be used for D1.
Frequency Selection/Enable (CTL)
The MAX8505 includes a frequency selection circuit to
allow it to run at 500kHz or 1MHz. The operating fre-
quency is selected through a control input, CTL, which
has three input threshold ranges that are ratiometric to
the input supply voltage. When CTL is driven to GND, it
acts like an enable pin, switching the output off. When
the CTL input is driven to >0.8 V
CC
, the MAX8505 is
enabled with 1MHz switching. When the CTL input is
between 0.55 V
CC
and 0.7 V
CC
, the part operates
at 500kHz. When the CTL input is <0.45 x V
CC
, the
device is in shutdown.
Soft-Start
To reduce input transient currents during startup, a pro-
grammable soft-start is provided. The soft-start time is
given by:
A minimum capacitance of 0.01µF at REF is recom-
mended to reduce the susceptibility to switching noise.
Power-OK (POK)
The MAX8505 also includes an open-drain POK output
that indicates when the regulator output is within ±12%
of its nominal output. If the output voltage moves
outside this range, the POK output is pulled to ground.
Since this comparator has no hysteresis on either
threshold, a 50µs delay time is added to prevent the
POK output from chattering between states. The POK
should be pulled to V
IN
or another supply voltage less
than 5.5V through a resistor.
UVLO
If V
CC
drops below +2.25V, the UVLO circuit inhibits
switching. Once V
CC
rises above +2.35V, the UVLO
clears, and the soft-start sequence activates.
Thermal Protection
Thermal-overload protection limits total power dissipa-
tion in the device. When the junction temperature
exceeds T
J
= +170°C, a thermal sensor forces the
device into shutdown, allowing the die to cool. The ther-
mal sensor turns the device on again after the junction
temperature cools by 20°C, resulting in a pulsed output
during continuous overload conditions. Following a
thermal-shutdown condition, the soft-start sequence
begins anew.
Design Procedure
Duty Cycle
The equation below shows how to calculate the result-
ing duty cycle when series losses from the inductor and
internal switches are accounted for:
where V
OUT
= output voltage; V
IN
= input voltage;
I
OUT
= output current (3A maximum); R
L
= ESR of the
inductor; R
NHS
= on-resistance of the high-side switch;
and R
NLS
= on-resistance of the low-side switch.
.
