Datasheet

18
LTC1624
APPLICATIONS INFORMATION
WUU
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Positive-to-Negative Converter: Output Voltage
Programming
Setting the output voltage for a positive-to-negative con-
verter is different from other architectures since the feed-
back voltage is referenced to the LTC1624 ground pin and
the ground pin is referenced to –V
OUT
. The output voltage
is set by a resistive divider according to the following
formula:
VV
R
R
V
DC
DC
OUT IN
=+






≈−
−






119 1
1
21
.
The external resistive divider is connected to the output as
shown in Figure 8.
Positive-to-Negative Converter: Power
MOSFET Selection
One external N-channel power MOSFET must be selected
for use with the LTC1624 for the switch. As in step-down
applications the source of the power MOSFET is con-
nected to the Schottky diode and inductor. The peak-to-
peak gate drive levels are set by the INTV
CC
voltage. The
gate drive voltage is equal to approximately 5V for V
IN
>
5.6V and a logic level MOSFET can be used. At V
IN
voltages
below 5V the INTV
CC
voltage is equal to V
IN
– 0.6V and a
sublogic level MOSFET should be used.
Selection criteria for the power MOSFET include the “ON”
resistance R
DS(ON)
, reverse transfer capacitance C
RSS
,
input voltage and maximum output current. When the
LTC1624 is operating in continuous mode the duty cycle
for the MOSFET is given by:
Main
V
VV
D
OUT D
Switch Duty Cycle =
V
V
OUT
IN
+
++
with V
OUT
 being the absolute value of V
OUT
.
The MOSFET power dissipation and maximum switch
current are given by:
that the voltage across C1 is constant such that V
C1
= V
IN
at full load over the entire V
IN
range. Assuming the enegry
storage in the coupling capacitor C1 must be equal to the
enegry stored in L1, the minimum capacitance of C1 is:
C
LI V
V
MIN
OUT OUT
IN MIN
1
1
22
4
()
()
=
()( )
SEPIC Converter: Duty Cycle Limitations
The minimum on-time of 450ns sets a limit on how high
an input-to-output ratio can be tolerated while not skip-
ping cycles. This only impacts designs when very low
output voltages (V
OUT
< 2.5V) are needed. Note that a
SEPIC converter would not be appropriate at these low
output voltages. The maximum input voltage is (remem-
ber not to exceed the absolute maximum limit of 36V):
V
IN(MAX)
= 10.1V
OUT
+ 5V For DC > 9%
Positive-to-Negative Converter Applications
The LTC1624 can also be used as a positive-to-negative
converter with a grounded inductor shown in Figure 8.
Since the LTC1624 requires a positive feedback signal
relative to device ground, Pin 4 must be tied to the
regulated negative output. A resistive divider from the
negative output to ground sets the output voltage.
Remember not to exceed maximum V
IN
ratings V
IN
+
V
OUT
 ≤ 36V.
P
MAIN
= I
SW MAX
×
I
OUT MAX
I + δ R
DS ON
+
 k V
IN MAX
+ V
OUT
1.85
C
RSS
200kHz

()
()
()
( )
()
()( )
()
{
{
Figure 8. Positive-to-Negative Converter
+
+
SENSE
–

I
TH
/RUN
V
FB

GND

V
IN

BOOST
TG
SW
LTC1624
1000pF
8
7
6
5
1
2
3
4
100pF
C
C
R
C
D1
C
B
R1
R2
C
OUT
M1
L1
R
SENSE
C
IN
–V
OUT
V
IN
1624 F08