MAX767EAP(1994) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX767EAP
(Rev.:1994)

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

Maxim Integrated 

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

Power-Supply Controller

_____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 appropri-

ate 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 dif-

ferent 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 out-

put power, high efficiency, and low noise. Good layout

includes the use of a ground plane, appropriate com-

ponent 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 perfor-

mance. 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 cir-

cuit board, trace resistance can contribute signifi-

cant 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 pos-

sible 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 resis-

tive 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 cop-

per 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 stan-

dard 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 compo-

nents and can exceed 10A. The 3.3V output is generat-

ed by a current-mode, pulse-width-modulation (PWM)

step-down regulator. The PWM regulator operates at

either 200kHz or 300kHz, with a corresponding trade-

off between somewhat higher efficiency (200kHz) and

smaller external component size (300kHz). The

MAX767 also has a 3.3V, 5mA reference voltage. Fault-

protection circuitry shuts off the output should the refer-

ence lose regulation or the input voltage go below 4V

(nominally).

External components for the MAX767 include two N-

channel 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 cur-

rent-sense resistor sets the maximum current limit, pre-

venting 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.

6 _______________________________________________________________________________________

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