MAX767 Maxim, MAX767 Datasheet - Page 6

• Manufacturer Part Number: MAX767
• Description: 5V-to-3.3V / Synchronous / Step-Down Power-Supply Controller
• Manufacturer: Maxim

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.
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-
2. Because the sense resistance values are similar to
3. Place the LX node components N1, N2, L1, and D2
4. The input filter capacitor C1 should be less than
5V-to-3.3V, Synchronous, Step-Down
Power-Supply Controller
6
_____Standard Application Circuits
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.
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.
as close together as possible. This reduces resis-
tive and switching losses and confines noise due to
ground inductance.
10mm away from N1’s drain. The connecting cop-
per trace carries large currents and must be at least
2mm wide, preferably 5mm.
_______________________________________________________________________________________
Layout and Grounding
5. Keep the gate connections to the MOSFETs short
6. To achieve good shielding, it is best to keep all
7. Connect the GND and PGND pins directly to the
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.
_______________Detailed Description
for low inductance (less than 20mm long and more
than 0.5mm wide) to ensure clean switching.
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.
ground plane, which should ideally be an inner
layer of a multilayer board.