LT1766 Linear Technology, LT1766 Datasheet - Page 17

• Manufacturer Part Number: LT1766
• Description: 5.5V to 60V 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
rating. A ground plane should always be used under the
switcher circuitry to prevent interplane coupling and over-
all noise.
The V
possible from the switch and boost nodes. The LT1766
pinout has been designed to aid in this. The ground for
these components should be separated from the switch
current path. Failure to do so will result in poor stability or
subharmonic like oscillation.
Board layout also has a significant effect on thermal
resistance. Pins 1, 8, 9 and 16, GND, are a continuous
copper plate that runs under the LT1766 die. This is the
best thermal path for heat out of the package. Reducing
the thermal resistance from Pins 1, 8, 9 and 16 onto the
board will reduce die temperature and increase the power
capability of the LT1766. This is achieved by providing as
much copper area as possible around these pins. Adding
multiple solder filled feedthroughs under and around
these four corner pins to the ground plane will also help.
Similar treatment to the catch diode and coil terminations
will reduce any additional heating effects. For the FE
C
and FB components should be kept as far away as
U
U
MINIMIZE LT1766
C3-D1 LOOP
GND
V
IN
W
GROUND PINS (4 CORNERS) FOR
GOOD THERMAL CONDUCTIVITY
PLACE FEEDTHROUGH AROUND
D1
C3
U
Figure 6. Suggested Layout
L1
1
2
3
4
5
6
7
8
GND
SW
V
BOOST
GND
C2
IN
LT1766
D2
BIAS
GND
GND
FB
V
package, the exposed pad (Pin 17) should be soldered to
the copper ground plane underneath the device.
PARASITIC RESONANCE
Resonance or “ringing” may sometimes be seen on the
switch node (see Figure 7). Very high frequency ringing
following switch rise time is caused by switch/diode/input
capacitor lead inductance and diode capacitance. Schot-
tky diodes have very high “Q” junction capacitance that
can ring for many cycles when excited at high frequency.
If total lead length for the input capacitor, diode and
switch path is 1 inch, the inductance will be approximately
25nH. At switch off, this will produce a spike across the
NPN output device in addition to the input voltage. At
higher currents this spike can be in the order of 10V to 20V
or higher with a poor layout, potentially exceeding the
absolute max switch voltage. The path around switch,
catch diode and input capacitor must be kept as short as
possible to ensure reliable operation. When looking at this,
a >100MHz oscilloscope must be used, and waveforms
should be observed on the leads of the package. This
C
16
15
14
13
12
11
10
9
SHDN
SYNC
R1
C
FB
C1
GROUND PLANE
R
C
CONNECT TO
C
C
R2
C
F
KEEP FB AND V
AWAY FROM HIGH FREQUENCY,
HIGH CURRENT COMPONENTS
V
GND
KELVIN SENSE
OUT
V
OUT
LT1766/LT1766-5
1766 F06
FOR THE FE PACKAGE, THE
EXPOSED PAD (PIN 17) SHOULD
BE PROPERLY SOLDERED TO
THE GROUND PLANE.
NOTE: BOOST AND BIAS
COPPER TRACES ARE ON
A SEPARATE LAYER FROM
THE GROUND PLANE
C
COMPONENTS
17
1766fa