Datasheet

11
LT1738
1738fa
APPLICATIO S I FOR ATIO
WUU
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OPERATIO
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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
R
T
is used to charge and discharge the capacitor C
T
. 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 C
T
is chosen by:
CnF
f kHz R k
T
T
()
()•()
=
Ω
2180
where f is the desired oscillator frequency in kHz. For R
T
equal to 16.9k, this simplifies to:
CnF
f kHz
T
()
()
=
129
e.g., C
T
= 1.29nF for f = 100kHz
Nominally R
T
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.
Table 1. Safety and Protection Features
FEATURE FUNCTION EFFECT on GATE DRIVER SLEW CONTROL EFFECT on V
C
, SS
Maximum Current Fault Turn Off FET at Maximum Immediately Goes Low Overridden None
Switch Current (V
SENSE
= 0.1)
Short-Circuit Fault Turn Off FET and Reset V
C
Immediately Goes Low Overridden Discharge V
C
, SS
for Short-Circuit (V
SENSE
= 0.22) to GND
Overvoltage Fault Turn Off Driver If FB > V
REG
+ 0.22V Immediately Goes Low Overridden None
(Output Overvoltage)
GCL Clamp Set Max Gate Voltage to Prevent Limits Max Voltage None None
FET Gate Breakdown
Gate Drive Disable Gate Drive When V
IN
Immediately Goes Low Overridden None
Undervoltage Lockout Is Too Low. Set Via GCL Pin
Thermal Shutdown Turn Off Driver If Chip Immediately Goes Low Overridden None
Temperature Is Too Hot
V
IN
Undervoltage Lockout Disable Part When V
IN
≅ 2.55V Immediately Goes Low Overridden None
Gate Drive Source and Sink Current Limit Limit Gate Drive Current Limit Drive Current None None
V5 Source/Sink Current Limit Limit Current from V5 None None None
Shutdown Disable Part When SHDN <1.3V