參數(shù)資料
型號(hào)： LT3579IUFD-1#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING REGULATOR, PQCC20
封裝： 4 X 5 MM, LEAD FREE, PLASTIC, MO-220, QFN-20
文件頁(yè)數(shù)： 2/40頁(yè)
文件大小： 503K
代理商： LT3579IUFD-1#PBF
LT3579/LT3579-1
10
35791f
operaTion
OPERATION – OVERVIEW
The LT3579 uses a constant-frequency, current mode
controlschemetoprovideexcellentlineandloadregulation.
Thepart’sundervoltagelockout(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,andovertemperatureconditions.RefertotheBlock
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
above1.33V(typical)enablesthepart.TakingtheSHDNpin
below0.3Vshutsdownthechip,resultinginextremelylow
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.TheUVLOfunctionsetstheturnon/offoftheLT3579
at a desired input voltage (VINUVLO). Figure 3 shows how
a resistor divider (or single resistor) from VIN to the SHDN
pin can be used to set VINUVLO. RUVLO2 is optional. It may
be left out, in which case set it to infinite in the equation
below. For increased accuracy, set RUVLO2 ≤ 10k. Pick
RUVLO1 as follows:
RUVLO1 =
VINUVLO –1.33V
1.33V
RUVLO2
+ 11.6A
RUVLO2
(OPTIONAL)
1.33V
RUVLO1
35791 F03
VIN
ACTIVE/
LOCKOUT
GND
11.6A
AT 1.33V
–
+
SHDN
Figure 3. Configurable UVLO
The LT3579 also has internal UVLO circuitry that disables
the chip when VIN < 2.3V (typical).
Soft-Start of Switch Current
The soft-start circuitry provides for a gradual ramp-up of
theswitchcurrent(refertoCommandedSwitchCurrentvs.
SS 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 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
currentincreaseslinearlywithSSvoltage,withamaximum
current of 933A 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 2k. The soft turn on of the external PMOS
helps limit inrush current at start-up, making hot plugs
of the LT3579 feasible and safe.
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