LT3579 LINER [Linear Technology], LT3579 Datasheet - Page 10

• Manufacturer Part Number: LT3579
• Description: 6A Boost/Inverting DC/DC Converter with Fault Protection
• Manufacturer: LINER [Linear Technology]

OPERATION
LT3579/LT3579-1
OPERATION – OVERVIEW
The LT3579 uses a constant-frequency, current mode
control scheme to provide excellent line and load regulation.
The part’s undervoltage lockout (UVLO) function, together
with soft-start and frequency foldback, offers a controlled
means of starting up. Fault features are incorporated in
the LT3579 to aid in the detection of output shorts, over-
voltage, and overtemperature conditions. Refer to the Block
Diagram (Figure 1) and the State Diagram (Figure 2) for
the following description of the part’s operation.
OPERATION – START-UP
Several functions are provided to enable a very clean
start-up for the LT3579.
Precise Turn-On Voltage
The SHDN pin compares to an internal voltage reference to
give a precise turn on voltage level. Taking the SHDN pin
above 1.33V (typical) enables the part. Taking the SHDN pin
below 0.3V shuts down the chip, resulting in extremely low
quiescent current. The SHDN pin has 30mV of hysteresis
to protect against glitches and slow ramping.
Undervoltage-Lockout (UVLO)
The SHDN pin can also be used to create a confi gurable
UVLO. The UVLO function sets the turn on/off of the LT3579
at a desired input voltage (VIN
a resistor divider (or single resistor) from V
pin can be used to set VIN
be left out, in which case set it to infi nite in the equation
below. For increased accuracy, set R
R
10
UVLO1
R
UVLO
as follows:
1
=
⎛
⎜
⎜
⎝
R
VIN
1 33
UVLO
.
UVLO
V
2
⎞
⎟
⎟
⎠
– .
+
UVLO
1 33
11
.6 6 µA
UVLO
. R
V
UVLO2
). Figure 3 shows how
UVLO2
is optional. It may
IN
≤ 10kΩ. Pick
to the SHDN
The LT3579 also has internal UVLO circuitry that disables
the chip when V
Soft-Start of Switch Current
The soft-start circuitry provides for a gradual ramp-up of
the switch current (refer to Commanded Switch Current vs.
SS in Typical Performance Characteristics). When the part
is brought out of shutdown, the external SS capacitor is
fi rst discharged which resets the states of the logic circuits
in the chip. Then an integrated 250k resistor pulls the SS
pin to ~1.8V at a ramp rate set by the external capacitor
connected to the pin. Once SS gets to 1.8V, the CLKOUT
pin is enabled, and an internal regulator pulls the pin up
quickly to ~2.1V. Typical values for the external soft-start
capacitor range from 100nF to 1μF.
Soft-Start of External PMOS (if used)
The soft-start circuitry also gradually ramps up the GATE
pin pull-down current which allows an external PMOS
to slowly turn on (M1 in Block Diagram). The GATE pin
current increases linearly with SS voltage, with a maximum
current of 933μA when the SS voltage gets above 500mV.
Note that if the GATE pin voltage is less than 2V for SS
voltages exceeding 500mV, then the GATE pin impedance
to ground is 2kΩ. The soft turn on of the external PMOS
helps limit inrush current at start-up, making hot plugs
of the LT3579 feasible and safe.
V
IN
R
R
(OPTIONAL)
UVLO1
UVLO2
Figure 3. Confi gurable UVLO
IN
V
SHDN
GND
< 2.3V (typical).
IN
AT 1.33V
11.6μA
1.33V
–
+
LOCKOUT
ACTIVE/
3579 F03
35791f