LT1766EFE#TRPBF Linear Technology, LT1766EFE#TRPBF Datasheet - Page 10

• Manufacturer Part Number: LT1766EFE#TRPBF
• Description: IC REG SW HV 1.5A 200KHZ 16TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1766EFE#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.2 ~ 54 V
• Current - Output: 1.5A
• Frequency - Switching: 200kHz
• Voltage - Input: 5.5 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT1766/LT1766-5
APPLICATIONS INFORMATION
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1766 is used to set output
voltage and provide several overload protection features.
The fi rst part of this section deals with selecting resistors
to set output voltage and the remaining part talks about
foldback frequency and current limiting created by the FB
pin. Please read both parts before committing to a fi nal
design. The 5V fi xed output voltage part (LT1766-5) has
internal divider resistors and the FB pin is renamed SENSE,
connected directly to the output.
The suggested value for the output divider resistor (see
Figure 2) from FB to ground (R2) is 5k or less, and a
formula for R1 is shown below. The output voltage error
caused by ignoring the input bias current on the FB pin
is less than 0.25% with R2 = 5k. A table of standard 1%
values is shown in Table 1 for common output voltages.
Please read the following if divider resistors are increased
above the suggested values.
Table 1
More Than Just Voltage Feedback
The feedback pin is used for more than just output voltage
sensing. It also reduces switching frequency and current
limit when output voltage is very low (see the Frequency
Foldback graph in Typical Performance Characteristics).
This is done to control power dissipation in both the IC
and in the external diode and inductor during short-cir-
cuit conditions. A shorted output requires the switching
10
VOLTAGE
OUTPUT
R
(V)
3.3
1
10
12
15
3
5
6
8
=
R V
2
(
OUT
1 22
(kΩ)
4.99
4.99
4.99
4.75
4.47
4.32
.
4.12
4.12
R2
−
1 22
.
(NEAREST 1%)
)
(kΩ)
7.32
8.45
15.4
18.7
24.9
30.9
36.5
46.4
R1
DUE TO DISCREET 1%
% ERROR AT OUTPUT
RESISTOR STEPS
–0.43
–0.30
–0.54
–0.27
+0.32
+0.38
+0.20
+0.24
regulator to operate at very low duty cycles, and the
average current through the diode and inductor is equal
to the short-circuit current limit of the switch (typically 2A
for the LT1766, folding back to less than 1A). Minimum
switch on-time limitations would prevent the switcher
from attaining a suffi ciently low duty cycle if switching
frequency were maintained at 200kHz, so frequency is
reduced by about 5:1 when the feedback pin voltage drops
below 0.8V (see Frequency Foldback graph). This does
not affect operation with normal load conditions; one
simply sees a gear shift in switching frequency during
start-up as the output voltage rises.
In addition to lower switching frequency, the LT1766 also
operates at lower switch current limit when the feedback
pin voltage drops below 0.6V. Q2 in Figure 2 performs
this function by clamping the V
its normal 2.1V upper clamp level. This foldback current
limit greatly reduces power dissipation in the IC, diode
and inductor during short-circuit conditions. External syn-
chronization is also disabled to prevent interference with
foldback operation. Again, it is nearly transparent to the user
under normal load conditions. The only loads that may be
affected are current source loads which maintain full load
current with output voltage less than 50% of fi nal value.
In these rare situations the feedback pin can be clamped
above 0.6V with an external diode to defeat foldback cur-
rent limit. Caution: clamping the feedback pin means that
frequency shifting will also be defeated, so a combination
of high input voltage and dead shorted output may cause
the LT1766 to lose control of current limit.
The internal circuitry which forces reduced switching
frequency also causes current to fl ow out of the feedback
pin when output voltage is low. The equivalent circuitry is
shown in Figure 2. Q1 is completely off during normal op-
eration. If the FB pin falls below 0.8V, Q1 begins to conduct
current and reduces frequency at the rate of approximately
1.4kHz/μA. To ensure adequate frequency foldback (under
worst-case short-circuit conditions), the external divider
Thevinin resistance must be low enough to pull 115μA out
of the FB pin with 0.44V on the pin (R
result is that reductions in frequency and current limit are
affected by output voltage divider impedance. Although
C
pin to a voltage less than
DIV
≤ 3.8k). The net
1766fc