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General Description

The MAX8646 high-efficiency switching regulator deliv-

ers up to 6A load current at output voltages from 0.6V

to 0.9 x V

. The IC operates from 2.35V to 3.6V, mak-

ing it ideal for on-board point-of-load and postregula-

tion applications. Total output error is less than ±1%

over load, line, and temperature ranges.

The MAX8646 features fixed-frequency PWM mode

operation with a switching frequency range of 500kHz

to 2MHz set by an external resistor. High-frequency

operation allows for an all-ceramic capacitor design.

The high operating frequency also allows for small-size

external components.

The low-resistance on-chip nMOS switches ensure high

efficiency at heavy loads while minimizing critical induc-

tances, making the layout a much simpler task with

respect to discrete solutions. Following a simple layout

and footprint ensures first-pass success in new designs.

The MAX8646 comes with a high bandwidth (> 14MHz)

voltage-error amplifier. The voltage-mode control archi-

tecture and the voltage-error amplifier permit a type III

compensation scheme to be utilized to achieve maxi-

mum loop bandwidth, up to 20% of the switching fre-

quency. High loop bandwidth provides fast transient

response, resulting in less required output capacitance

and allowing for all-ceramic-capacitor designs.

The MAX8646 provides two tri-state logic inputs to

select one of nine preset output voltages. The preset

output voltages allow customers to achieve ±1% out-

put-voltage accuracy without using expensive 0.1%

resistors. In addition, the output voltage can be set to

any customer value by either using two external resis-

tors at the feedback with 0.6V internal reference or

applying an external reference voltage to the REFIN

input. The MAX8646 offers programmable soft-start

time using one capacitor to reduce input inrush current.

The MAX8646 is available in a lead-free, 24-pin, 4mm x

4mm, thin QFN package.

Applications

POLs

ASIC/CPU/DSP Core and I/O Voltages

DDR Power Supplies

Base-Station Power Supplies

Telecom and Networking Power Supplies

RAID Control Power Supplies

Features

o Internal 23mΩ R

DS(ON)

MOSFETs

o Continuous 6A Output Current

o ±1% Output Accuracy over Load, Line,

and Temperature

o Operates from 2.35V to 3.6V Supply

o Adjustable Output from 0.6V to (0.9 x V

IN)

o Soft-Start Reduces Inrush Supply Current

o 500kHz to 2MHz Adjustable Switching Frequency

o Compatible with Ceramic, Polymer, and

Electrolytic Output Capacitors

o VID-Selectable Output Voltages

0.6V, 0.7V, 0.8V, 1.0V, 1.2V, 1.5V, 1.8V, 2.0V,

and 2.5V

o Fully Protected Against Overcurrent and

Overtemperature

o Safe-Start into Prebiased Output

o Sink/Source Current in DDR Applications

o Lead-Free, 24-Pin, 4mm x 4mm Thin QFN Package

MAX8646

6A, 2MHz Step-Down Regulator

with Integrated Switches

________________________________________________________________

Maxim Integrated Products

OUTPUT

1.8V, 6A

INPUT

2.4V, 3.6V

BST

OUT

CTL1

CTL2

PGND

COMP

PWRGD

FREQ

REFIN

GND

PREBIAS

MAX8646

Typical Operating Circuit

Ordering Information

19-3971; Rev 2; 10/07

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,

or visit Maxim’s website at www.maxim-ic.com.

PART

TEMP

RANGE

PIN-PACKAGE

PKG

CODE

MAX8646ETG+ -40°C to +85°C

24 Thin QFN-EP*

4mm x 4mm

T2444-4

Denotes a lead-free package.

EP = Exposed pad.

Pin Configuration appears at end of data sheet.

EVALUATION KIT

AVAILABLE

Summary of content (16 pages)

PAGE 1
9-3971; Rev 2; 10/07 KIT ATION EVALU LE B A IL A AV 6A, 2MHz Step-Down Regulator with Integrated Switches Features The MAX8646 high-efficiency switching regulator delivers up to 6A load current at output voltages from 0.6V to 0.9 x VIN. The IC operates from 2.35V to 3.6V, making it ideal for on-board point-of-load and postregulation applications. Total output error is less than ±1% over load, line, and temperature ranges.
PAGE 2
MAX8646 6A, 2MHz Step-Down Regulator with Integrated Switches ABSOLUTE MAXIMUM RATINGS IN, VDD, PWRGD to GND ......................................-0.3V to +4.5V COMP, FB, REFIN, OUT, CTL_, EN, SS, FREQ to GND...................-0.3V to (VDD + 0.3V) LX Current (Note 1) .....................................................-8A to +8A BST to LX..................................................................-0.3V to +4V PGND to GND .......................................................-0.3V to +0.
PAGE 3
6A, 2MHz Step-Down Regulator with Integrated Switches (VIN = VDD = 3.3V, VFB = 0.5V, TA = -40°C to +85°C, typical values are at TA = +25°C, circuit of Figure 1, unless otherwise noted.) (Note 2) PARAMETER Error-Amplifier Unity-Gain Bandwidth Error-Amplifier Common-Mode Input Range Error-Amplifier Minimum Output Current FB Input Bias Current CONDITIONS Parallel 10kΩ, 40pF from COMP to GND (Note 3) MIN TYP 14 26 MAX MHz VDD = 2.35V to 2.6V 0 VDD - 1.65 VDD = 2.6V to 3.6V 0 VDD - 1.
PAGE 4
MAX8646 6A, 2MHz Step-Down Regulator with Integrated Switches ELECTRICAL CHARACTERISTICS (continued) (VIN = VDD = 3.3V, VFB = 0.5V, TA = -40°C to +85°C, typical values are at TA = +25°C, circuit of Figure 1, unless otherwise noted.) (Note 2) PARAMETER CONDITIONS Frequency Range MIN TYP 500 LX Minimum Off-Time VIN = 2.5V to 3.3V LX Maximum Duty Cycle RFREQ = 50kΩ, VIN = 2.5V to 3.3V LX Minimum On-Time RFREQ = 50kΩ, VIN = 2.5V to 3.
PAGE 5
6A, 2MHz Step-Down Regulator with Integrated Switches EFFICIENCY vs. OUTPUT CURRENT EFFICIENCY vs. OUTPUT CURRENT EFFICIENCY vs. OUTPUT CURRENT 95 95 90 VOUT = 2.5V 70 VOUT = 1.8V VOUT = 1.2V 60 85 80 VOUT = 1.5V VOUT = 1.8V 75 VOUT = 1.2V 70 50 VIN = VDD = 2.5V 60 10 0.1 FREQUENCY vs. INPUT VOLTAGE - 40°C + 25°C + 85°C 1650 1500 1350 1200 - 40°C + 25°C + 85°C 1050 VOUT = 1.2V VIN = 2.5V VDD = 3.3V 60 1 OUTPUT CURRENT (A) 0.
PAGE 6
Typical Operating Characteristics (continued) (Typical values are: VIN = VDD = 3.3V; VOUT = 1.8V, RFREQ = 50kΩ, IOUT = 6A, and TA = +25°C, unless otherwise noted.) CURRENT LIMIT vs. OUTPUT VOLTAGE INPUT CURRENT vs. INPUT VOLTAGE 7 8 CURRENT LIMIT (A) 6 5 4 3 2 1V/div 0V 7 6 IOUT 5A/div 5 0A 4 IIN 3 VEN = 0V 1 VOUT 1A/div 0A 2 2.2 2.4 2.6 2.8 3.0 3.2 INPUT VOLTAGE (V) 3.4 0.5 3.6 RMS INPUT CURRENT DURING SHORT-CIRCUIT vs. INPUT VOLTAGE (C4 = 0.022µF) 0.40 0.35 0.30 0.25 0.20 0.15 0.
PAGE 7
6A, 2MHz Step-Down Regulator with Integrated Switches PIN NAME FUNCTION 1 PREBIAS 2 VDD Supply Voltage and Bypass Input. Connect VDD to IN with a 10Ω resistor. Connect a 1µF ceramic capacitor from VDD to GND. 3, 4 CTL1, CTL2 Preset Output Voltage Selection Input. CTL1 and CTL2 set the output voltage to one of nine preset voltages. See Table 1 for preset voltages. 5 REFIN External Reference Input. Connect REFIN to SS to use the internal 0.6V reference.
PAGE 8
6A, 2MHz Step-Down Regulator with Integrated Switches MAX8646 Block Diagram VDD MAX8646 UVLO CIRCUITRY SHUTDOWN CONTROL EN CURRENT-LIMIT COMPARATOR BIAS GENERATOR LX ILIM THRESHOLD BST IN VOLTAGE REFERENCE SS BST CAPACITOR CHARGING SWITCH SOFT-START CONTROL LOGIC LX IN THERMAL SHUTDOWN REFIN PGND OUT ERROR AMPLIFIER 8kΩ PWM COMPARATOR PREBIAS FB CTL1 CTL2 VID VOLTAGECONTROL CIRCUITRY FREQ 1VP-P OSCILLATOR COMP SHDN COMP LOW DETECTOR FB 0.
PAGE 9
6A, 2MHz Step-Down Regulator with Integrated Switches INPUT 2.4V TO 3.6V IN C6 22µF C7 0.1µF VDD R5 10Ω BST C10 0.1µF L1 0.47µH MAX8646 OUTPUT 1.8V, 6A LX VDD C5 1µF OUT C8 22µF x2 C3 680pF CTL2 C9 0.01µF R3 432Ω CTL1 PGND EN FB FREQ C2 1500pF R2 3.57kΩ REFIN R4 49.9kΩ SS C1 33pF C4 0.022µF COMP PREBIAS GND VDD R1 20kΩ PWRGD Figure 1. MAX8646 1MHz, All-Ceramic-Capacitor Design with VOUT = 1.
PAGE 10
MAX8646 6A, 2MHz Step-Down Regulator with Integrated Switches voltage exceeds the VCOMP 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 11A 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.
PAGE 11
6A, 2MHz Step-Down Regulator with Integrated Switches Table 1. CTL1 and CTL2 Output Voltage Selection CTL1 CTL2 GND GND 0.6 Thermal Protection VDD VDD 0.7 Thermal-overload protection limits total power dissipation in the device. When the junction temperature exceeds TJ = +165°C, a thermal sensor forces the device into shutdown, allowing the die to cool.
PAGE 12
MAX8646 6A, 2MHz Step-Down Regulator with Integrated Switches lating the output voltage back to its predetermined value. The controller response time depends on the closed-loop bandwidth. A higher bandwidth yields a faster response time, preventing the output from deviating further from its regulating value. See the Compensation Design section for more details. Input-Capacitor Selection The input capacitor reduces the current peaks drawn from the input power supply and reduces switching noise in the IC.
PAGE 13
6A, 2MHz Step-Down Regulator with Integrated Switches L VOUT LX COUT MAX8646 R3 OUT R2 R1 = C3 C3 = FB CTL1 C1 R1 CTL2 COMP R4 VOUT LX COUT R2 OUT R3 8kΩ C3 FB VOLTAGE SELECT CTL1 R1 C1 COMP CTL2 L x CO x (RO + ESR) x L x CO x (RO + ESR) 0.8 x C1 1 0.8 x R 3 R2 = a) EXTERNAL RESISTOR DIVIDER MAX8646 x RL + RO RL + RO Setting the second compensation pole, f P2_EA , at fZ_ESR yields: C2 L 1 C2 b) INTERNAL PRESET VOLTAGES Figure 3.
PAGE 14
OPEN-LOOP GAIN THIRD POLE DOUBLE POLE GAIN (dB) SECOND POLE 1) Connect input and output capacitors to the power ground plane; connect all other capacitors to the signal ground plane. 2) Place capacitors on VDD, VIN, and SS as close as possible to the IC and its corresponding pin using direct traces. Keep power ground plane (connected to PGND) and signal ground plane (connected to GND) separate. 3) Keep the high-current paths as short and wide as possible.
PAGE 15
6A, 2MHz Step-Down Regulator with Integrated Switches 24L QFN THIN.EPS ______________________________________________________________________________________ 15 MAX8646 Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.
PAGE 16
MAX8646 6A, 2MHz Step-Down Regulator with Integrated Switches Package Information (continued) (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.) Revision History Pages changed at Rev 1: 1, 5, 6, 7, 10–14 Pages changed at Rev 2: 1, 2, 4, 8, 9, 13, 15, 16 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product.