LT1766EFE#TRPBF Linear Technology, LT1766EFE#TRPBF Datasheet - Page 26

• 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: -

APPLICATIONS INFORMATION
LT1766/LT1766-5
Example: with V
V
OUTPUT DIVIDER
Refer to Applications Information Feedback Pin Functions
to calculate R1 and R2 for the (negative) output voltage
(from Table 1).
Inductor Value
The criteria for choosing the inductor is typically based on
ensuring that peak switch current rating is not exceeded.
This gives the lowest value of inductance that can be
used, but in some cases (lower output load currents) it
may give a value that creates unnecessarily high output
ripple voltage.
The diffi culty in calculating the minimum inductor size
needed is that you must fi rst decide whether the switcher
will be in continuous or discontinuous mode at the critical
point where switch current reaches 1.5A. The fi rst step is
to use the following formula to calculate the load current
above which the switcher must use continuous mode. If
your load current is less than this, use the discontinuous
26
F
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
I
V
V
V
0.3 = Switch voltage drop at 1.5A
5.5V TO
= 0.63V, I
INPUT
* INCREASE L1 TO 30μH OR 60μH FOR HIGHER CURRENT APPLICATIONS.
P
†
IN
OUT
F
SEE APPLICATIONS INFORMATION
FOR V
PATH OF D2 IS REQUIRED TO ENSURE BOOST PIN MAXIMUM RATING IS
NOT EXCEEDED. SEE APPLICATIONS INFORMATION (BOOST PIN VOLTAGE)
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
= Maximum rated switch current
48V
= Catch diode forward voltage
2.2μF
100V
= Minimum input voltage
CER
†
C3
= Output voltage
IN
> 44V AND V
Figure 15. Positive-to-Negative Converter
P
= 1.5A: I
V
IN
GND
IN(MIN)
C
OUT
LT1766
BOOST
F
= –12V, ADDITIONAL VOLTAGE DROP IN THE
MAX
V
V
C
SW
= 5.5V, V
FB
R
C
C
C
= 0.280A.
D1
10MQO60N
C2
0.33μF
OUT
1N4148W
D2
†
18μH
= 12V, L = 18μH,
L1*
R2
4.99k
44.2k
R1
+
C1
100μF
25V
TANT
OUTPUT**
–12V, 0.25A
1766 F15
mode formula to calculate minimum inductor needed. If
load current is higher, use the continuous mode formula.
Output current where continuous mode is needed:
Minimum inductor discontinuous mode:
Minimum inductor continuous mode:
For a 40V to –12V converter using the LT1766 with peak
switch current of 1.5A and a catch diode of 0.63V:
For a load current of 0.25A, this says that discontinuous
mode can be used and the minimum inductor needed is
found from:
In practice, the inductor should be increased by about
30% over the calculated minimum to handle losses and
variations in value. This suggests a minimum inductor of
18μH for this application.
Ripple Current in the Input and Output Capacitors
Positive-to-negative converters have high ripple current
in the input capacitor. For long capacitor lifetime, the RMS
value of this current must be less than the high frequency
ripple current rating of the capacitor. The following formula
will give an approximate value for RMS ripple current. This
formula assumes continuous mode and large inductor
value . Small inductors will give somewhat higher ripple
current, especially in discontinuous mode. The exact
formulas are very complex and appear in Application
I
I
L
L
L
CONT
CONT
MIN
MIN
MIN
=
=
=
>
>
(
2
2
200 10
(
( )(
V
f V
4
4 40 12 40 12 0 63
2 12 0 25
OUT
( )( )
(
(
( )( . )
f I
V
•
IN
IN
P
)(
+
+
+
I
3
2
OUT
( ) ( . )
V
V
)( . )
40 1 5
OUT
OUT
)(
1 5
(
)
V
2
IN
)(
)
2
⎡
⎢
⎣
(
) ( )
+
V
I
2
V
P
IN
=
IN
I
P
–
2
13 3
)(
+
I
2
OUT
+
V
V
.
OUT
OUT
μ
. )
⎛
⎜
⎝
H
1
)
+
+
=
V
(
F
V
0 573
)
OUT
.
V
IN
+
A
V
F
1766fc
)
⎞
⎟
⎠
⎤
⎥
⎦