Datasheet

LT1766/LT1766-5
27
1766fc
Note 44, pages 29 and 30. For our purposes here a fudge
factor (ff) is used. The value for ff is about 1.2 for higher
load currents and L ≥15μH. It increases to about 2.0 for
smaller inductors at lower load currents.
Input Capacitor I ff I
V
V
RMS OUT
OUT
IN
= ()( )
ff = 1.2 to 2.0
The output capacitor ripple current for the positive-to-
negative converter is similar to that for a typical buck
regulator—it is a triangular waveform with peak-to-peak
value equal to the peak-to-peak triangular waveform of the
inductor. The low output ripple design in Figure 15 places
the input capacitor between V
IN
and the regulated negative
output. This placement of the input capacitor signifi cantly
reduces the size required for the output capacitor (versus
placing the input capacitor between V
IN
and ground).
The peak-to-peak ripple current in both the inductor and
output capacitor (assuming continuous mode) is:
I
P-P
P-P
=
==
+
++
=
DC V
fL
DC Duty Cycle
VV
VVV
I RMS
I
IN
OUT F
OUT IN F
COUT
•
•
()
12
The output ripple voltage for this confi guration is as low
as the typical buck regulator based predominantly on the
inductor’s triangular peak-to-peak ripple current and the
ESR of the chosen capacitor (see Output Ripple Voltage
in Applications Information).
Diode Current
Average diode current is equal to load current. Peak diode
current will be considerably higher.
Peak diode current:
Continuous Mode
I
VV
V
VV
LfV V
Discontinuous Mode
IV
Lf
OUT
IN OUT
IN
IN OUT
IN OUT
OUT OUT
=
+
+
+
=
()()()
()()( )
()( )
()()
2
2
Keep in mind that during start-up and output overloads,
average diode current may be much higher than with nor-
mal loads. Care should be used if diodes rated less than
1A are used, especially if continuous overload conditions
must be tolerated.
BOOST Pin Voltage
To ensure that the BOOST pin voltage does not exceed its
absolute maximum rating of 68V with respect to device
GND pin voltage, care should be taken in the generation of
boost voltage. For the conventional method of generating
boost voltage, shown in Figure 1, the voltage at the BOOST
pin during switch on time is approximately given by:
V
BOOST
(GND pin) = (V
IN
– V
GNDPIN
) + V
C2
where:
V
C2
= (D2+) – V
D2
– (D1+) + V
D1
= voltage across the boost capacitor
For the positive-to-negative converter shown in Figure 15,
the conventional Buck output node is grounded (D2+) = 0V
and the catch diode (D1+) is connected to the negative
output = V
OUT
= –12V. Absolute maximum ratings should
also be observed with the GND pin now at –12V. It can be
seen that for V
D1
= V
D2
:
V
C2
= (D2+) – (D1+) = |V
OUT
| = 12V
The maximum V
IN
voltage allowed for the device (GND
pin at –12V) is 48V.
The maximum V
IN
voltage allowed without exceeding the
BOOST pin voltage absolute maximum rating is given by:
V
IN(MAX)
= Boost (Max) + (V
GNDPIN
) – V
C2
V
IN(MAX)
= 68 + (–12) – 12 = 44V
To increase usable V
IN
voltage, V
C2
must be reduced. This
can be achieved by placing a zener diode V
Z1
(anode at
C2+) in series with D2.
Note: A maximum limit on V
Z1
must be observed to
ensure a minimum V
C2
is maintained on the boost
capacitor; referred to as V
BOOST(MIN)
in the Electrical
Characteristics.
APPLICATIONS INFORMATION