lt3579iufd-trpbf Linear Technology Corporation, lt3579iufd-trpbf Datasheet - Page 22

• Manufacturer Part Number: lt3579iufd-trpbf
• Description: Lt3579/lt3579-1 6a Boost/inverting Dc/dc Converter With Fault Protection Features
• Manufacturer: Linear Technology Corporation

LT3579/LT3579-1
APPLICATIONS INFORMATION
Use the following equations to calculate the FB resistor
for 2-phase converters:
Note that the CLKOUT pin on the LT3579-1 runs at a fi xed
duty cycle of ~50%. If monitoring the die temperature is
desired, the slave IC can be a LT3579.
It is possible to use the LT3579-1 in a multiphase converter
of more than 2 phases. Consult the LTC Applications
Engineering Department for more information.
CHARGE PUMP AIDED REGULATORS
Designing charge pumps with the LT3579 can offer effi cient
solutions with fewer components than traditional circuits
because of the master/slave switch confi guration on the
IC. The current in the master switch (SW1) is sensed by
the current comparator (A4 in Block Diagram), but the
current in the slave switch (SW2) is not. Note that the slave
switch, SW2, operates in phase with SW1. This method
of operation by the master/slave switches can offer the
following benefi ts to charge pump designs:
• The slave switch, by not performing a current sense
• The master switch, immune from the capacitor current
• Since the slave switch can sustain large current spikes,
22
operation like the master switch, can sustain fairly
large current spikes when the fl ying capacitors charge
up. Since this current spike fl ows through SW2, it
does not affect the operation of the current comparator
(A4 in Block Diagram).
spike, can sense the inductor current more accurately.
the diodes that feed current into the fl ying capacitors
do not need current limiting resistors, leading to
effi ciency and thermal improvements.
R
R
FB
FB
=
=
⎛
⎜
⎝
⎛
⎜
⎝
V
|
V
OUT
2 83 3
2 83 3
OUT
•
•
– .
|
+
1 215
.
.
9
μA
μA
mV
V
⎞
⎟
⎠
; Inverting Multiphase
⎞
⎟
⎠
; Boost or SEPIC
Multiphase Converter
Converter
V
High V
The LT3579 can be used in a charge-pump topology (refer
to Figure 16), multiplying the output of an inductive boost
converter. The master switch (SW1) can be used to drive
the inductive boost converter, while the slave switch (SW2)
can be used to drive one or more charge pump stages. This
topology is useful for high voltage applications including
VFD Bias Supplies.
Single Inductor Inverting Topology
If there is a need to use just 1 inductor to generate a
negative output voltage whose magnitude is greater than
V
Figure 15) can be used. Since the master and slave switches
are isolated by an external Schottky diode, the current spike
through C1 will fl ow through the slave switch, thereby
preventing the current comparator (A4 in Block Diagram)
from falsely tripping. Output disconnect is inherently built
into the single inductor topology.
PWR
12V
IN
V
5V
IN
, the Single Inductor Inverting topology (shown in
500k
OUT
V
Figure 13. 2-Phase Converters Using LT3579-1
4.7μF
OUT1
10μF
4.7μF
Charge Pump Topology
10μF
100k
21.5k
5k
4.7μH
86.6k
86.6k
4.7μH
V
FAULT
SHDN
RT
V
FAULT
SHDN
RT
SYNC
SYNC
IN
IN
SW1 SW2
SW1 SW2
LT3579-1
LT3579-1
MASTER
SLAVE
GND
GND
CLKOUT
CLKOUT
4.7μF
GATE
GATE
SS
SS
V
V
FB
FB
2
C
C
0.22μF
0.22μF
4.7μF
2
V
PWR
47pF
V
137k
OUT1
6.4k
7k
2.2nF
3579 F13
35791f
V
24V
3.7A,
89W
4.7μF
OUT
2