LTC1624
U
W U U
APPLICATIONS INFORMATION
Positive-to-Negative Converter: Output Voltage
Programming
that the voltage across C1 is constant such that VC1 = VIN
at full load over the entire VIN 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:
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 –VOUT. The output voltage
is set by a resistive divider according to the following
formula:
2
) (
2
)
L1I
V
OUT
(
OUT
C1
=
MIN
(
)
4
)
V
IN MIN
(
R1
R2
DC
1−DC
SEPIC Converter: Duty Cycle Limitations
V
= 1.19V 1+
≈ −V
IN
OUT
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 (VOUT < 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):
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 INTVCC voltage. The
gate drive voltage is equal to approximately 5V for VIN >
5.6VandalogiclevelMOSFETcanbeused. AtVIN voltages
below 5V the INTVCC voltage is equal to VIN – 0.6V and a
sublogic level MOSFET should be used.
VIN(MAX) = 10.1VOUT + 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.
Selection criteria for the power MOSFET include the “ON”
resistance RDS(ON), reverse transfer capacitance CRSS
input voltage and maximum output current. When the
LTC1624 is operating in continuous mode the duty cycle
for the MOSFET is given by:
Remember not to exceed maximum VIN ratings VIN
VOUT ≤ 36V.
+
,
1000pF
8
V
IN
V
+ V
D
1
2
3
4
OUT
–
V
IN
SENSE
Main Switch Duty Cycle =
+
+
C
C
R
R
C
IN
SENSE
C
V + V
+ V
D
7
IN
OUT
BOOST
LTC1624
I
/RUN
TH
6
5
M1
TG
with VOUT being the absolute value of VOUT.
V
FB
C
B
100pF
L1
The MOSFET power dissipation and maximum switch
current are given by:
SW
GND
D1
R1
P
MAIN
= I
×
C
OUT
SW MAX
(
)
R2
I
I + δ R
+
(
)
OUT MAX
DS ON
(
)
(
)
{
–V
OUT
1.85
1624 F08
k V
(
+ V
C
(
200kHz
)
)(
RSS
)
IN MAX
OUT
Figure 8. Positive-to-Negative Converter
(
)
{
18
Linear [ Linear ]
