LT1766 Linear Technology, LT1766 Datasheet - Page 9

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

APPLICATIO S I FOR ATIO
should be used if resistors are increased beyond the
suggested values and short-circuit conditions occur with
high input voltage. High frequency pickup will increase
and the protection accorded by frequency and current
foldback will decrease.
CHOOSING THE INDUCTOR
For most applications, the output inductor will fall into the
range of 15 H to 100 H. Lower values are chosen to
reduce physical size of the inductor. Higher values allow
more output current because they reduce peak current
seen by the LT1766 switch, which has a 1.5A limit. Higher
values also reduce output ripple voltage.
When choosing an inductor you will need to consider
output ripple voltage, maximum load current, peak induc-
tor current and fault current in the inductor. In addition,
other factors such as core and copper losses, allowable
component height, EMI, saturation and cost should also
be considered. The following procedure is suggested as a
way of handling these somewhat complicated and con-
flicting requirements.
Output Ripple Voltage
Figure 3 shows a typical output ripple voltage waveform
for the LT1766. Ripple voltage is determined by ripple
current (I
frequency impedance of the output capacitor. The follow-
ing equations will help in choosing the required inductor
LP-P
) through the inductor and the high
U
LT1766
V
U
C
Q2
GND
AMPLIFIER
W
ERROR
TO SYNC CIRCUIT
Figure 2. Frequency and Current Limit Foldback
TO FREQUENCY
SHIFTING
+
–
U
1.4V
1.2V
R3
1k
BUFFER
Q1
R4
2k
value to achieve a desirable output ripple voltage level. If
output ripple voltage is of less importance, the subse-
quent suggestions in Peak Inductor and Fault Current
and EMI will additionally help in the selection of the
inductor value.
Peak-to-peak output ripple voltage is the sum of a triwave
(created by peak-to-peak ripple current (I
and a square wave (created by parasitic inductance (ESL)
and ripple current slew rate). Capacitive reactance is
assumed to be small compared to ESR or ESL.
40mV/DIV
0.5A/DIV
V
RIPPLE
Figure 3. LT1766 Ripple Voltage Waveform
V
V
L = 47 H
C = 100 F, 10V, 0.1
IN
OUT
V
= 40V
SW
FB
= 5V
I
LP P -
L1
ESR
2.5 s/DIV
R1
R2
5k
+
LT1766/LT1766-5
1766 F02
C1
ESL
OUTPUT
5V
dt
dI
1766 F03
LP-P
V
V
INDUCTOR CURRENT
AT I
INDUCTOR CURRENT
AT I
OUT
OUT
OUT
OUT
AT I
AT I
) times ESR)
= 1A
= 0.1A
OUT
OUT
= 1A
= 0.1A
1766fa
9