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

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

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.

3A, 1MHz, 1% Accurate, Internal Switch

Step-Down Regulator with Power-OK

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9

Pin Description

PIN

NAME

FUNCTION

1, 3, 14, 16

LX

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.

2, 4

IN

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.

5

BST

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 10

Ω resistor and bypass to

GND with 0.1µF.

7

POK

Power-OK Output. Open-drain output of a window comparator that pulls POK low when the FB pin is

outside the 0.8V

±12% range.

8

CTL

9

COMP

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.

10

FB

Feedback Input. This pin regulates to 0.8V. Use an external resistive-divider from the output to set the

output voltage.

11

REF

Place a capacitor at this pin to set the soft-start time. This pin goes to 0V when the part is shut down.

12

GND

Ground

13, 15

PGND

Power Ground. Connect this pin to GND at a single point.