LT1766 Linear Technology, LT1766 Datasheet - Page 25

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

APPLICATIO S I FOR ATIO
load currents and L 15 H. It increases to about 2.0 for
smaller inductors at lower load currents.
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
output. This placement of the input capacitor significantly
reduces the size required for the output capacitor (versus
placing the input capacitor between V
The peak-to-peak ripple current in both the inductor and
output capacitor (assuming continuous mode) is:
The output ripple voltage for this configuration 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:
Input Capacitor I
I
DC Duty Cycle
I
I
Discontinuous Mode
Continuous Mode
P-P
COUT
OUT
(
V
(
DC V
RMS
IN
f L
V
•
•
IN
V
)
IN
OUT
I
U
P-P
)
RMS
12
V
2
OUT
( )( )(
L f V
U
V
OUT
(
( )(
V
ff I
IN
IN
2
V
(
and the regulated negative
I
)(
IN
OUT
OUT
IN
V
V
( )( )
OUT
L f
F
W
)(
)
V
V
F
V
IN
OUT
)
V
OUT
V
OUT
and ground).
IN
)
)
U
Keep in mind that during start-up and output overloads,
average diode current may be much higher than with
normal 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:
where:
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
also be observed with the GND pin now at –12V. It can be
seen that for V
The maximum V
at –12V) is 48V.
The maximum V
BOOST pin voltage absolute maximum rating is given by:
To increase usable V
can be achieved by placing a zener diode V
C2+) in series with D2.
Note: A maximum limit on V
ensure a minimum V
capacitor; referred to as “V
Characteristics.
V
V
V
V
V
BOOST
C2
C2
IN(MAX)
IN(MAX)
= (D2+) – V
= (D2+) – (D1+) = |V
= voltage across the “boost” capacitor
OUT
(GND pin) = (V
= Boost (Max) + (V
= 68 + (–12) – 12 = 44V
= –12V. Absolute maximum ratings should
D1
IN
IN
= V
voltage allowed for the device (GND pin
D2
voltage allowed without exceeding the
IN
D2
– (D1+) + V
voltage, V
C2
:
IN
is maintained on the “boost”
LT1766/LT1766-5
OUT
– V
BOOST(MIN)
GNDPIN
Z1
| = 12V
GNDPIN
C2
D1
must be observed to
must be reduced. This
) – V
) + V
” in the Electrical
C2
C2
Z1
(anode at
25
1766fa