LT1738 Просмотр технического описания (PDF) - Linear Technology
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LT1738 Datasheet PDF : 20 Pages
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OPERATIO
LT1738
Table 1. Safety and Protection Features
FEATURE
FUNCTION
Maximum Current Fault
Short-Circuit Fault
Overvoltage Fault
Turn Off FET at Maximum
Switch Current (VSENSE = 0.1)
Turn Off FET and Reset VC
for Short-Circuit (VSENSE = 0.22)
Turn Off Driver If FB > VREG + 0.22V
(Output Overvoltage)
GCL Clamp
Set Max Gate Voltage to Prevent
FET Gate Breakdown
Gate Drive
Undervoltage Lockout
Disable Gate Drive When VIN
Is Too Low. Set Via GCL Pin
Thermal Shutdown
Turn Off Driver If Chip
Temperature Is Too Hot
VIN Undervoltage Lockout
Gate Drive Source and Sink Current Limit
Disable Part When VIN ≅ 2.55V
Limit Gate Drive Current
V5 Source/Sink Current Limit
Limit Current from V5
Shutdown
Disable Part When SHDN <1.3V
EFFECT on GATE DRIVER
Immediately Goes Low
Immediately Goes Low
Immediately Goes Low
Limits Max Voltage
Immediately Goes Low
Immediately Goes Low
Immediately Goes Low
Limit Drive Current
None
SLEW CONTROL EFFECT on VC, SS
Overridden
None
Overridden
Overridden
Discharge VC, SS
to GND
None
None
None
Overridden
None
Overridden
None
Overridden
None
None
None
None
None
APPLICATIO S I FOR ATIO
Reducing EMI from switching power supplies has tradi-
tionally invoked fear in designers. Many switchers are
designed solely on efficiency and as such produce wave-
forms filled with high frequency harmonics that then
propagate through the rest of the system.
The LT1738 provides control over two of the more impor-
tant variables for controlling EMI with switching inductive
loads: switch voltage slew rate and switch current slew
rate. The use of this part will reduce noise and EMI over
conventional switch mode controllers. Because these
variables are under control, a supply built with this part will
exhibit far less tendency to create EMI and less chance of
encountering problems during production.
It is beyond the scope of this data sheet to get into EMI
fundamentals. Application Note 70 contains much infor-
mation concerning noise in switching regulators and
should be consulted.
Oscillator Frequency
The oscillator determines the switching frequency and
therefore the fundamental positioning of all harmonics.
The use of good quality external components is important
to ensure oscillator frequency stability. The oscillator is of
a sawtooth design. A current defined by external resistor
RT is used to charge and discharge the capacitor CT . The
discharge rate is approximately ten times the charge rate.
By allowing the user to have control over both compo-
nents, trimming of oscillator frequency can be more easily
achieved.
The external capacitance CT is chosen by:
C T (nF) =
2180
f(kHz)•RT (kΩ)
where f is the desired oscillator frequency in kHz. For RT
equal to 16.9k, this simplifies to:
C T (nF) =
129
f(kHz)
e.g., CT = 1.29nF for f = 100kHz
Nominally RT should be 16.9k. Since it sets up current, its
temperature coefficient should be selected to compliment
the capacitor. Ideally, both should have low temperature
coefficients.
1738fa
11
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