Rainbow Electronics MAX767 User Manual

________________General Description

The MAX767 is a high-efficiency, synchronous buck
controller IC dedicated to converting a fixed 5V supply
into a tightly regulated 3.3V output. Two key features set
this device apart from similar, low-voltage step-down
switching regulators: high operating frequency and all
N-channel construction in the application circuit. The
300kHz operating frequency results in very small, low­cost external surface-mount components.

The inductor, at 3.3µH for 5A, is physically at least five
times smaller than inductors found in competing solu­tions. All N-channel construction and synchronous rectifi­cation result in reduced cost and highest efficiency.
Efficiency exceeds 90% over a wide range of loading,
eliminating the need for heatsinking. Output capacitance
requirements are low, reducing board space and cost.

The MAX767 is a monolithic BiCMOS IC available in
20-pin SSOP packages. For other fixed output voltages
and package options, please consult the factory.

________________________Applications

Local 5V-to-3.3V DC-DC Conversion

Microprocessor Daughterboards

Power Supplies up to 10A or More

____________________________Features

♦ >90% Efficiency
♦ 700µA Quiescent Supply Current
♦ 120µA Standby Supply Current
♦ 4.5V-to-5.5V Input Range
♦ Low-Cost Application Circuit
♦ All N-Channel Switches
♦ Small External Components
♦ Tiny Shrink-Small-Outline Package (SSOP)
♦ Predesigned Applications:

Standard 5V to 3.3V DC-DC Converters up to 10A
High-Accuracy Pentium P54C VR-Spec Supply

♦ Fixed Output Voltages Available:

3.3V (Standard)

3.45V (High-Speed Pentium™)

3.6V (PowerPC™)

_______________Ordering Information

MAX767

5V-to-3.3V, Synchronous, Step-Down

Power-Supply Controller

________________________________________________________________ Maxim Integrated Products 1

__________________Pin Configuration

OUTPUT

3.3V

AT 5A

INPUT

4.5V TO 5.5V

LX

DL

GND

MAX767

V

CC

REF

3.3µH

BST

DH

PGND

CS

FB

ON

_________Typical Application Circuit

19-0224; Rev 3; 7/00

PART TEMP. RANGE

PIN­PACKAGE

MAX767CAP 0°C to +70°C 20 SSOP

MAX767RCAP 0°C to +70°C 20 SSOP

MAX767SCAP 0°C to +70°C 20 SSOP

Pentium is a trademark of Intel. PowerPC is a trademark of IBM.

MAX767C/D 0°C to +70°C Dice*

REF
TOL

±1.8%

±1.8%

±1.8%

—

Ordering Information continued at end of data sheet.

* Contact factory for dice specifications.

V

OUT

(V)

3.3

3.45

3.6

—

MAX767TCAP 0°C to +70°C 20 SSOP ±1.2% 3.3

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.

TOP VIEW

CS

1

SS

2

ON

GND

GND

GND

GND

REF

SYNC

V

3

MAX767

4

5

6

7

8

9

CC

10

SSOP

FB

20

DH

19

LX

18

BST

17

DL

16

V

15

CC

V

14

CC

PGND

13

N.C.

12

GND

11

MAX767

5V-to-3.3V, Synchronous, Step-Down
Power-Supply Controller

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

VCCto GND.................................................................-0.3V, +7V

PGND to GND ........................................................................±2V

BST to GND ...............................................................-0.3V, +15V

LX to BST.....................................................................-7V, +0.3V

Inputs/Outputs to GND

(ON, REF, SYNC, CS, FB, SS) .....................-0.3V, V

CC

+ 0.3V

DL to PGND .....................................................-0.3V, V

CC

+ 0.3V

DH to LX...........................................................-0.3V, BST + 0.3V

REF Short to GND.......................................................Momentary

REF Current.........................................................................20mA

Continuous Power Dissipation (T

A

= +70°C)

20-Pin SSOP (derate 8.00mW/°C above +70°C) ..........640mW

Operating Temperature Ranges:

MAX767CAP/MAX767_CAP.................................0°C to +70°C

MAX767EAP/MAX767_EAP ..............................-40°C to +85°C

Lead Temperature (soldering, 10s) .................................+300°C

PARAMETER

Oscillator Frequency

DH Sink/Source Current

MIN TYP MAX

1

260 300 340

UNITS

SS Source Current

A

2.50 4 6.5

(BST - LX) = 4.5V, DH = 2V

DL On Resistance

µA

Current-Limit Voltage

200

7

80 100 120 mV

VCCFault Lockout Voltage

Ω

3.80 4.20

kHz

V

Line Regulation

High or low

0.1 %

Oscillator SYNC Range

SS Fault Sink Current

DH On Resistance

2 mA

7

3.24 3.30 3.36

Ω

240 350 kHz

SYNC High Pulse Width

VCCStandby Current

High or low, (BST - LX) = 4.5V

120 200

200

µA

VCCQuiescent Current

ns

0.7 1.0 mA

SYNC Low Pulse Width 200 ns

SYNC Rise/Fall Time

Output Voltage (FB)

200 ns

Oscillator Maximum Duty Cycle

VCCInput Supply Range

89 92

4.5 5.5 V

95

%

Input Low Voltage

3.17 3.35 3.46

V

0.8 V

Input High Voltage

2.40

3.32 3.50 3.60

V

CC

- 0.5

V

Input Current ±1 µA

DL Sink/Source Current 1 A

CONDITIONS

SYNC = 3.3V

SYNC = 0V or 5V

CS - FB

Falling edge, hysteresis = 1%

VCC= 4.5V to 5.5V

MAX767, MAX767R, MAX767S

ON = 0V, VCC= 5.5V

FB = CS = 3.5V

Not tested

SYNC = 3.3V

SYNC = 0V

SYNC, ON

ON

0mV < (CS - FB) < 80mV,

4.5V < V

CC

< 5.5V
(includes load and
line regulation)

SYNC

SYNC, ON = 0V or 5V

DL = 2V

Load Regulation 2.5 %(CS - FB) = 0mV to 80mV

3.46 3.65 3.75

MAX767R

MAX767S

MAX767, MAX767T

ELECTRICAL CHARACTERISTICS

(VCC= ON = 5V, GND = PGND = SYNC = 0V, I

REF

= 0mA, TA= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at TA= +25°C.)

Reference Voltage (REF)

3.26 3.30 3.34

V

MAX767T

MAX767

5V-to-3.3V, Synchronous, Step-Down

Power-Supply Controller

_______________________________________________________________________________________ 3

__________________________________________Typical Operating Characteristics

(Circuit of Figure 1 (5A configuration), VIN= 5V, oscillator frequency = 300kHz, TA= +25°C, unless otherwise noted.)

EFFICIENCY vs. OUTPUT CURRENT

(1.5A CIRCUIT)

100

90

80

70

EFFICIENCY (%)

60

50

0.001 0.1 10

0.01 1
OUTPUT CURRENT (A)

EFFICIENCY vs. OUTPUT CURRENT

(7A CIRCUIT)

100

90

80

MAX767-01

EFFICIENCY (%)

MAX767-04

EFFICIENCY vs. OUTPUT CURRENT

(3A CIRCUIT)

100

90

80

70

60

50

0.001 0.1 10

0.01 1
OUTPUT CURRENT (A)

EFFICIENCY vs. OUTPUT CURRENT

(10A CIRCUIT)

100

90

80

100

MAX767-02

EFFICIENCY (%)

1000

MAX767-05

100

EFFICIENCY vs. OUTPUT CURRENT

(5A CIRCUIT)

90

80

70

60

50

0.001 0.1 10

0.01 1
OUTPUT CURRENT (A)

SWITCHING FREQUENCY vs.

PERCENT OF FULL LOAD

SYNC = REF (300kHz)

10

MAX767-03

MAX767-06

70

EFFICIENCY (%)

60

50

0.001 0.1 10

0.01 1
OUTPUT CURRENT (A)

IDLE-MODE WAVEFORMS

I

= 300mA

LOAD

5µs/div

70

EFFICIENCY (%)

60

50

0.001 0.1 10

0.01 1
OUTPUT CURRENT (A)

3.3V OUTPUT
50mV/div, AC COUPLED

LX
5V/div

I

LOAD

1

0.1

SWITCHING FREQUENCY (kHz)

0.01

0.001 1 100

PWM-MODE WAVEFORMS

= 5A

1µs/div

0.01 0.1 10

LOAD CURRENT (% FULL LOAD)

3.3V OUTPUT
50mV/div, AC COUPLED

LX
5V/div

MAX767

5V-to-3.3V, Synchronous, Step-Down
Power-Supply Controller

4 _______________________________________________________________________________________

_____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 1 (5A configuration), VIN= 5V, oscillator frequency = 300kHz, TA= +25°C, unless otherwise noted.)

1.5A CIRCUIT LOAD-TRANSIENT RESPONSE

200µs/div

1.5A

3.3V OUTPUT
50mV/div
AC-COUPLED

0A

LOAD CURRENT

3A CIRCUIT LOAD-TRANSIENT RESPONSE

200µs/div

3A

3.3V OUTPUT
50mV/div
AC-COUPLED

0A

LOAD CURRENT

5A CIRCUIT LOAD-TRANSIENT RESPONSE

200µs/div

5A

3.3V OUTPUT
50mV/div
AC-COUPLED

0A

LOAD CURRENT

10A CIRCUIT LOAD-TRANSIENT RESPONSE

200µs/div

10A

3.3V OUTPUT
50mV/div
AC-COUPLED

0A

LOAD CURRENT

7A CIRCUIT LOAD-TRANSIENT RESPONSE

200µs/div

7A

3.3V OUTPUT
50mV/div
AC-COUPLED

0A

LOAD CURRENT

MAX767

5V-to-3.3V, Synchronous, Step-Down

Power-Supply Controller

_______________________________________________________________________________________ 5

______________________________________________________________Pin Description

16

NAME FUNCTION

1

CS Current-sense input: +100mV = nominal current-limit level referred to FB.

2 SS Soft-start input. Ramp time to full current limit is 1ms/nF of capacitance to GND.

3 ON

ON/O—F—F–control input to disable the PWM. Tie directly to VCCfor automatic start-up.

PIN

DL Gate-drive output for the low-side synchronous rectifier MOSFET

4–7, 11 GND Low-current analog ground. Feedback reference point for the output.

8 REF 3.3V internal reference output. Bypass to GND with 0.22µF minimum capacitor.

9 SYNC

Oscillator control/synchronization input. Connect to VCCor GND for 200kHz; connect to REF for
300kHz. For external clock synchronization in the 240kHz to 350kHz range, a high-to-low transition
causes a new cycle to start.

10, 14, 15 V

CC

Supply voltage input: 4.5V to 5.5V

17 BST Boost capacitor connection (0.1µF)

12 N.C. No internal connection

13 PGND Power ground

18 LX Inductor connection. Can swing 2V below GND without latchup.

19 DH Gate-drive output for the high-side MOSFET

20 FB Feedback and current-sense input for the PWM

Figure 1. Standard Application Circuit

R2

INPUT

4.5V TO 5.5V

SHUTDOWN

ON/OFF

C5

(OPTIONAL)

C6

0.22µF

0.01µF

4.7µF

C4

ON

SS

SYNC

REF

V

CC

MAX767

GND

10

BST

PGND

Ω

DH

LX

DL

CS

FB

D1
SMALL­SIGNAL
SCHOTTKY

C3

N1

0.1µF

D2

N2

C1

OUTPUT

L1

R1

3.3V

C2

MAX767

_____Standard Application Circuits

This data sheet shows five predesigned circuits with
output current capabilities from 1.5A to 10A. Many
users will find one of these standard circuits appropri­ate for their needs. If a standard circuit is used, the
remainder of this data sheet (Detailed Description and
Applications Information and Design Procedure) can
be bypassed.

Figure 1 shows the Standard Application Circuit. Table 1
gives component values and part numbers for five dif­ferent implementations of this circuit: 1.5A, 3A, 5A, 7A,
and 10A output currents.

Each of these circuits is designed to deliver the full
rated output load current over the temperature range
listed. In addition, each will withstand a short circuit of
several seconds duration from the output to ground. If
the circuit must withstand a continuous short circuit,
refer to the Short-Circuit Duration section for the
required changes.

Layout and Grounding

Good layout is necessary to achieve the designed out­put power, high efficiency, and low noise. Good layout
includes the use of a ground plane, appropriate com­ponent placement, and correct routing of traces using
appropriate trace widths. The following points are in
order of decreasing importance.

1. A ground plane is essential for optimum perfor­mance. In most applications, the circuit will be
located on a multilayer board and full use of the four
or more copper layers is recommended. Use the
top and bottom layers for interconnections and the
inner layers for an uninterrupted ground plane.

2. Because the sense resistance values are similar to
a few centimeters of narrow traces on a printed cir­cuit board, trace resistance can contribute signifi­cant errors. To prevent this, Kelvin connect CS and
FB to the sense resistor; i.e., use separate traces
not carrying any of the inductor or load current, as
shown in Figure 2. These signals must be carefully
shielded from DH, DL, BST, and the LX node.

Important: place the sense resistor as close as pos­sible to and no further than 10mm from the MAX767.

3. Place the LX node components N1, N2, L1, and D2
as close together as possible. This reduces resis­tive and switching losses and confines noise due to
ground inductance.

4. The input filter capacitor C1 should be less than
10mm away from N1’s drain. The connecting cop­per trace carries large currents and must be at least
2mm wide, preferably 5mm.

5. Keep the gate connections to the MOSFETs short
for low inductance (less than 20mm long and more
than 0.5mm wide) to ensure clean switching.

6. To achieve good shielding, it is best to keep all
switching signals (MOSFET gate drives DH and DL,
BST, and the LX node) on one side of the board
and all sensitive nodes (CS, FB, and REF) on the
other side.

7. Connect the GND and PGND pins directly to the
ground plane, which should ideally be an inner
layer of a multilayer board.

_______________Detailed Description

Note: The remainder of this document contains the
detailed information necessary to design a circuit that
differs substantially from the five standard application
circuits. If you are using one of the predesigned stan­dard circuits, the following sections are provided only
for your reading pleasure.

The MAX767 converts a 4.5V to 5.5V input to a 3.3V
output. Its load capability depends on external compo­nents and can exceed 10A. The 3.3V output is generat­ed by a current-mode, pulse-width-modulation (PWM)
step-down regulator. The PWM regulator operates at
either 200kHz or 300kHz, with a corresponding trade­off between somewhat higher efficiency (200kHz) and
smaller external component size (300kHz). The
MAX767 also has a 3.3V, 5mA reference voltage. Fault­protection circuitry shuts off the output should the refer­ence lose regulation or the input voltage go below 4V
(nominally).

External components for the MAX767 include two N­channel MOSFETs, a rectifier, and an LC output filter.
The gate-drive signal for the high-side MOSFET, which
must exceed the input voltage, is provided by a boost
circuit that uses a 0.1µF capacitor. The synchronous
rectifier keeps efficiency high by clamping the voltage
across the rectifier diode. An external low-value cur­rent-sense resistor sets the maximum current limit, pre­venting excessive inductor current during start-up or
under short-circuit conditions. An optional external
capacitor sets the programmable soft-start, reducing
in-rush surge currents upon start-up and providing
adjustable power-up time.

The PWM regulator is a direct-summing type, lacking a
traditional integrator-type error amplifier and the phase
shift associated with it. It therefore does not require
external feedback-compensation components, as long
as you follow the ESR guidelines in the Applications
Information and Design Procedure sections.

5V-to-3.3V, Synchronous, Step-Down
Power-Supply Controller

6 _______________________________________________________________________________________