ML4865
DESIGN CONSIDERATIONS (Continued)
SHUTDOWN
If an external Schottky is required, look for a device with
a voltage rating of 20V or greater. The average forward
current rating should be at least 500mA, and the forward
voltage should be 600mV or less. Suitable Schottky
rectifiers can be obtained from the following vendors:
The SHDN pin is a high impedance input and is noise
sensitive. Either drive the SHDN input from a low
impedance source or bypass the pin to GND with a 10nF
ceramic capacitor.
Diodes, Inc B120
(805) 446-4800
(310) 322-3331
(602) 897-5056
SENSE
Int’l Rectifier10BQ040
The SENSE pin should be left open or bypassed to ground
for normal operation. The output can be set to voltages
lower than the preset value by adding a resistor divider.
The output voltage can be determined from the following
equation:
Motorola
MBR0520L
LAYOUT
Good layout practices will ensure the proper operation of
the ML4865. Some layout guidelines follow:
R1 R2
R2
VOUT 2.42
(V)
(3)
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4865
where R1 and R2 are connected as shown in Figure 2. The
value of R2 should be 1MW or less to minimize bias
current errors. Choose an appropriate value of R2 and
calculate the value of R1.
• Use short trace lengths from the inductor to the V and
L1
OUT
V
pins and from the output capacitor to the V
pin
L2
V
ꢀ
ꢃ
1
OUT
R1 R2
ꢂ
ꢅ
ꢁ 2.42 ꢄ (W)
(4)
• Use a single point ground for the ML4865 ground pin,
and the input and output capacitors
EXTERNAL SCHOTTKY RECTIFIER
• Separate the ground for the converter circuitry from the
ground of the load circuitry and connect at a single
point
Due to excessive power dissipation, an external Schottky
rectifier is required when operating at input voltages
above 6V. Even for applications where the input voltage is
below 6V, the use of an external rectifier may be
necessary to achive efficiency or output current
requirements.
A sample layout is shown in Figure 8.
Figure 8. Sample ML4865 Layout
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
