LT3581 Datasheet, PDF (10/36 Pages) Linear Technology – 3.3A Boost/Inverting DC/DC Converter with Fault Protection

LT3581
Operation
Operation – Overview
The LT3581 uses a constant-frequency, current mode con-
trol 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
LT3581 to aid in the detection of output shorts, over-volt-
age, 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 LT3581:
Precise Turn-On Voltage
The SHDN pin is compared 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 300mV 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 configurable
UVLO. The UVLO function sets the turn on/off of the LT3581
at a desired input voltage (V_INUVLO). Figure 3 shows how a
resistor divider (or single resistor) from V_IN to the SHDN
pin can be used to set V_INUVLO. R_UVLO2 is optional. It may
be left out, in which case set it to infinite in the equation
below. For increased accuracy, set R_UVLO2 ≤ 10k. Pick
R_UVLO1 as follows:
R_UVLO1
=


V_INUVLO – 1.33V
1.33V
R_UVLO2


+
11.6µA
V_IN
V_IN
R_UVLO1
1.33V –
SHDN
+
R_UVLO2
(OPTIONAL)
11.6µA
AT 1.33V
GND
ACTIVE/
LOCKOUT
3581 F03
Figure 3. Configurable UVLO
The LT3581 also has internal UVLO circuitry that disables
the chip when V_IN < 2.3V (typical).
Soft-Start of Switch Current
The soft-start circuitry provides for a gradual ramp-up
of the switch current (refer to Commanded Current Limit
vs SS Voltage in Typical Performance Characteristics).
When the part is brought out of shutdown, the external
SS capacitor is first discharged which resets the states
of the logic circuits in the chip. Then an integrated 250k
resistor pulls the SS pin to ~1.8V. The ramp rate of the SS
pin voltage is set by this 250k resistor and the external
capacitor connected to this 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 the 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 LT3581s feasible and safe.
3581f
10