Detailed Description
The MAX8505 is a high-efficiency synchronous buck
regulator capable of delivering up to 3A of output
current. It operates in PWM mode at a high fixed
frequency of 500kHz or 1MHz, thereby reducing
external component size. The MAX8505 operates from
a 2.6V to 5.5V input voltage and can produce an output
voltage from 0.8V to 0.85 V
IN
.
Controller Block Function
The MAX8505 step-down converter uses a PWM
current-mode control scheme. An open-loop comparator
compares the voltage-feedback error signal against the
sum of the amplified current-sense signal and the slope
compensation ramp. At each rising edge of the internal
clock, the internal high-side MOSFET turns on until the
PWM comparator trips. During this on-time, current ramps
up through the inductor, sourcing current to the output
and storing energy in the inductor. The current-mode
feedback system regulates the peak inductor current as a
function of the output-voltage error signal. Since the aver-
age inductor current is nearly the same as the peak
inductor current, the circuit acts as a switch-mode
transconductance amplifier. To preserve inner-loop
stability and eliminate inductor staircasing, a slope-
compensation ramp is summed into the main PWM com-
parator. During the second half of the cycle, the internal
high-side N-channel MOSFET turns off, and the internal
low-side N-channel MOSFET turns on. The inductor
releases the stored energy as its current ramps down
while still providing current to the output. The output
capacitor stores charge when the inductor current
exceeds the load current, and discharges when the
inductor current is lower, smoothing the voltage across
the load. Under overload conditions, when the inductor
current exceeds the current limit (see the
Current Limit
section), the high-side MOSFET does not turn on at the
rising edge of the clock and the low-side MOSFET
remains on to let the inductor current ramp down.
Current Sense
An internal current-sense amplifier produces a current
signal proportional to the voltage generated by the high-
side MOSFET on-resistance and the inductor current
(R
DS(ON)
I
LX
). The amplified current-sense signal and
the internal slope-compensation signal are summed
together into the comparator’s inverting input. The PWM
comparator turns off the internal high-side MOSFET
when this sum exceeds the feedback voltage from the
voltage-error amplifier.
MAX8505
3A, 1MHz, 1% Accurate, Internal Switch
Step-Down Regulator with Power-OK
_______________________________________________________________________________________ 9
Pin Description
Inductor Connection. Connect an inductor between these pins and the regulator output. All LX pins must
be connected together externally. Connect a 3300pF ceramic capacitor from LX to PGND.
Power-Supply Inputs. Ranges from 2.6V to 5.5V. Bypass with two ceramic 22µF capacitors to GND. All IN
pins must be connected together externally.
Bootstrapped Voltage Input. High-side driver supply pin. Bypass to LX with a 0.1µF capacitor. Charged
from IN with an external Schottky diode.
Supply Voltage and Gate-Drive Supply for Low-Side Driver. Decouple with a 10Ω resistor and bypass to
GND with 0.1µF.
Power-OK Output. Open-drain output of a window comparator that pulls POK low when the FB pin is
Output Control. When at GND, the regulator is off. When at V
, the regulator is operating at 1MHz. For a
.
Regulator Loop Compensation. Connect a series RC network to GND. This pin is pulled to GND when the
output is shut down, or in UVLO or thermal shutdown.
Feedback Input. This pin regulates to 0.8V. Use an external resistive-divider from the output to set the
output voltage.
