Datasheet

LT1766/LT1766-5
26
1766fc
APPLICATIONS INFORMATION
I
P
= Maximum rated switch current
V
IN
= Minimum input voltage
V
OUT
= Output voltage
V
F
= Catch diode forward voltage
0.3 = Switch voltage drop at 1.5A
Example: with V
IN(MIN)
= 5.5V, V
OUT
= 12V, L = 18μH,
V
F
= 0.63V, I
P
= 1.5A: I
MAX
= 0.280A.
OUTPUT DIVIDER
Refer to Applications Information Feedback Pin Functions
to calculate R1 and R2 for the (negative) output voltage
(from Table 1).
mode formula to calculate minimum inductor needed. If
load current is higher, use the continuous mode formula.
Output current where continuous mode is needed:
I
VI
VV VV V
CONT
IN P
IN OUT IN OUT F
>
+++
()()
()( )
22
4
Minimum inductor discontinuous mode:
L
VI
fI
MIN
OUT OUT
P
=
2
2
()()
()( )
Minimum inductor continuous mode:
L
VV
fV V I I
VV
V
MIN
IN OUT
IN OUT P OUT
OUT F
IN
=
++
+
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎤
⎦
⎥
()( )
()( ) –
()
21
For a 40V to –12V converter using the LT1766 with peak
switch current of 1.5A and a catch diode of 0.63V:
IA
CONT
>
+++
=
()(.)
()( .)
.
40 1 5
440124012063
0 573
22
For a load current of 0.25A, this says that discontinuous
mode can be used and the minimum inductor needed is
found from:
LH
MIN
==μ
212 025
200 10 1 5
13 3
32
()(.)
(•)(.)
.
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
OUTPUT**
–12V, 0.25A
INPUT
†
5.5V TO
48V
1766 F15
C2
0.33μF
C
C
R
C
D1
10MQO60N
R1
44.2k
C1
100μF
25V
TANT
C3
2.2μF
100V
CER
D2
†
1N4148W
L1*
18μH
C
F
BOOST
LT1766
V
IN
V
SW
FB
GND
V
C
R2
4.99k
* INCREASE L1 TO 30μH OR 60μH FOR HIGHER CURRENT APPLICATIONS.
SEE APPLICATIONS INFORMATION
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
†
FOR V
IN
> 44V AND V
OUT
= –12V, ADDITIONAL VOLTAGE DROP IN THE
PATH OF D2 IS REQUIRED TO ENSURE BOOST PIN MAXIMUM RATING IS
NOT EXCEEDED. SEE APPLICATIONS INFORMATION (BOOST PIN VOLTAGE)
+
Figure 15. Positive-to-Negative Converter
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