MAX767 Maxim, MAX767 Datasheet - Page 11

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

The SYNC input controls the oscillator frequency.
Connecting SYNC to GND or to V
operation; connecting it to REF selects 300kHz opera-
tion. SYNC can also be driven with an external 240kHz
to 350kHz CMOS/TTL source to synchronize the inter-
nal oscillator. Normally, 300kHz operation is chosen to
minimize the inductor and output filter capacitor sizes,
but 200kHz operation may be chosen for a small (about
1%) increase in efficiency at heavy loads.
The internal 3.3V bandgap reference (REF) remains
active, even when the switching regulator is turned off.
It can furnish up to 5mA, and can be used to supply
memory keep-alive power or for other purposes.
Bypass REF to GND with 0.22µF, plus 1µF/mA of load
current.
Most users will be able to work with one of the standard
application circuits; others may want to implement a
circuit with an output current rating that lies between or
beyond the standard values.
If you want an output current level that lies between two
of the standard application circuits, you can interpolate
many of the component values from the values given
for the two circuits. These components include the
input and output filter capacitors, the inductor, and the
sense resistor. The capacitors must meet ESR and rip-
ple current requirements (see Input Filter Capacitor and
Output Filter Capacitor sections). The inductor must
meet the required current rating (see Inductor section).
You may use the rectifier and MOSFETs specified for
the circuit with the greater output current capability, or
choose a new rectifier and MOSFETs according to the
requirements detailed in the Rectifier and MOSFET
Switches sections. For more complete information, or
for output currents in excess of 10A, refer to the design
information in the following sections.
Three inductor parameters are required: the inductance
value (L), the peak inductor current (I
coil resistance (R
__________________Design Procedure
L1 = ______________
Applications Information and
f x I
OUT
1.32
L
______________________________________________________________________________________
). The inductance is:
x LIR
5V-to-3.3V, Synchronous, Step-Down
Oscillator Frequency
Internal Reference
CC
LPEAK
selects 200kHz
Inductor, L1
), and the
Power-Supply Controller
where:
A higher LIR value allows smaller inductance, but
results in higher losses and ripple.
The highest peak inductor current (I
DC load current (I
inductor current (I
current is typically chosen as 30% of the maximum DC
load current, so the peak inductor current is 1.15 x I
The peak inductor current at any load is given by:
The coil resistance should be as low as possible,
preferably in the low milliohms. The coil is effectively in
series with the load at all times, so the wire losses alone
are approximately:
In general, select a standard inductor that meets the L,
I
unavailable, choose a core with an LI
greater than L x I
will fit the core.
The current-sense resistor must carry the peak current
in the inductor, which exceeds the full DC load current.
The internal current limiting starts when the voltage
across the sense resistors exceeds 100mV nominally,
80mV minimum. Use the minimum value to ensure ade-
quate output current capability: R1 = 80mV / I
The low V
start-up under full load or with load transients from no-
load to full load. If the supply is subjected to these con-
ditions, reduce the sense resistor:
Since 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 the CS and FB pins to the
sense resistors; i.e., use separate traces not carrying
any of the inductor or load current, as shown in Figure 2.
LPEAK
f = switching frequency, normally 300kHz
I
LIR = ratio of inductor peak-to-peak AC
I
Power Loss = I
R1 = ———
, and R
OUT
LPEAK
IN
= maximum 3.3V DC load current (A)
/V
70mV
I
= I
LPEAK
OUT
current to average DC load current,
typically 0.3.
L
OUT
requirements. If a standard inductor is
LPEAK
ratio creates a potential problem with
LPP
OUT
Current-Sense Resistor, R1
+ __________
OUT
). The peak-to-peak AC inductor
) plus half the peak-to-peak AC
2 x f x L1
2
, and use the largest wire that
2
1.32
x R
L
LPEAK
2
) equals the
parameter
LPEAK
OUT
.
11
.