LT1766 Linear Technology, LT1766 Datasheet - Page 16

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

LT1766/LT1766-5
APPLICATIO S I FOR ATIO
Keep the connections from the resistors to the shutdown
pin short and make sure that interplane or surface capaci-
tance to the switching nodes are minimized. If high
resistor values are used, the shutdown pin should be
bypassed with a 1000pF capacitor to prevent coupling
problems from the switch node. If hysteresis is desired in
the undervoltage lockout point, a resistor R
added to the output node. Resistor values can be calcu-
lated from:
Example: output voltage is 5V, switching is to stop if input
voltage drops below 12V and should not restart unless
input rises back to 13.5V. V is therefore 1.5V and
V
SYNCHRONIZING
The SYNC input must pass from a logic level low, through
the maximum synchronization threshold with a duty cycle
between 10% and 90%. The input can be driven directly
from a logic level output. The synchronizing range is equal
to initial operating frequency up to 700kHz. This means
that minimum practical sync frequency is equal to the
worst-case high self-oscillating frequency (228kHz), not
the typical operating frequency of 200kHz. Caution should
be used when synchronizing above 265kHz because at
higher sync frequencies the amplitude of the internal slope
16
IN
25k suggested for R
V
R
R
R
R
= 12V. Let R
IN
V = Hysteresis in input voltage level
HI
FB
HI
FB
= Input voltage at which switching stops as input
voltage descends to trip level
R
25 10 41
25 12 2 38 1 5 5 1 1 5
116 5 1 5
R
LO IN
HI
k
k
2 24
k
.
V
V
2 38 25 5 5
LO
OUT
.
.
/ .
2 38
.
= 25k.
U
2 38
.
/
–
.
LO
V
116
387
R
k
U
. /
LO
V V
k
/
.
k
5 5
OUT
.
A
W
A
1
.
V
U
FB
can be
compensation used to prevent subharmonic switching is
reduced. This type of subharmonic switching only occurs
at input voltages less than twice output voltage. Higher
inductor values will tend to eliminate this problem. See
Frequency Compensation section for a discussion of an
entirely different cause of subharmonic switching before
assuming that the cause is insufficient slope compensa-
tion. Application Note 19 has more details on the theory
of slope compensation.
At power-up, when V
Figure 2, Q2), the sync function is disabled. This allows the
frequency foldback to operate in the shorted output con-
dition. During normal operation, switching frequency is
controlled by the internal oscillator until the FB pin reaches
0.6V, after which the SYNC pin becomes operational. If no
synchronization is required, this pin should be connected
to ground.
LAYOUT CONSIDERATIONS
As with all high frequency switchers, when considering
layout, care must be taken in order to achieve optimal elec-
trical, thermal and noise performance. For maximum effi-
ciency, switch rise and fall times are typically in the nano-
second range. To prevent noise both radiated and con-
ducted, the high speed switching current path, shown in
Figure 5, must be kept as short as possible. This is imple-
mented in the suggested layout of Figure 6. Shortening
this path will also reduce the parasitic trace inductance of
approximately 25nH/inch. At switch off, this parasitic in-
ductance produces a flyback spike across the LT1766
switch. When operating at higher currents and input volt-
ages, with poor layout, this spike can generate voltages
across the LT1766 that may exceed its absolute maximum
V
IN
Figure 5. High Speed Switching Path
C3
CIRCULATING
C
FREQUENCY
LT1766
is being clamped by the FB pin (see
PATH
HIGH
D1 C1
L1
LOAD
1766 F05
5V
1766fa