LT1246_15 Datasheet, PDF (1/12 Pages) Linear Technology – 1MHz Off-Line Current Mode PWM and DC/DC Converter

FEATURES
s _{Current Mode Operation to 1MHz}
s _{30ns Current Sense Delay}
s _{< 250µA Low Start-Up Current}
s _{Current Sense Leading Edge Blanking}
s _{Pin Compatible with UC1842}
s _{Undervoltage Lockout with Hysteresis}
s _{No Cross-Conduction Current}
s _{Trimmed Bandgap Reference}
s _{1A Totem Pole Output}
s _{Trimmed Oscillator Frequency and Sink Current}
s _{Active Pull-Down on Reference and Output During}
Undervoltage Lockout
s _{18V High Level Output Clamp}
APPLICATI S
s _{Off-Line Converters}
s _{DC/DC Converters}
Device
LT1246
LT1247
Start-Up
Threshold
16V
8.4V
Minimum
Operating
Voltage
10V
7.6V
Maximum
Duty Cycle
100%
100%
Replaces
UC1842
UC1843
LT1246/LT1247
1MHz Off-Line
Current Mode PWM
and DC/DC Converter
DESCRIPTIO
The LT^®1246/LT1247 are 8-pin, fixed frequency, current
mode, pulse width modulators. These devices are de-
signed to be improved plug compatible versions of the
industry standard UC1842 PWM circuit. The LT1246/
LT1247 are optimized for off-line and DC/DC converter
applications. They contain a temperature compensated
reference, high gain error amplifier, current sensing com-
parator, and a high current totem pole output stage ideally
suited to driving power MOSFETs. Start-up current has
been reduced to less than 250µA. Cross-conduction cur-
rent spikes in the totem pole output stage have been
eliminated, making 1MHz operation practical. Several new
features have been incorporated. Leading edge blanking
has been added to the current sense comparator. This
minimizes or eliminates the filter that is normally required.
Eliminating this filter allows the current sense loop to
operate with minimum delays. Trims have been added to
the oscillator circuit for both frequency and sink current,
and both of these parameters are tightly specified. The
output stage is clamped to a maximum V_OUT of 18V in the
on state. The output and the reference output are actively
pulled low during under-voltage lockout.
, LTC and LT are registered trademarks of Linear Technology Corporation.
BLOCK DIAGRA
R_T/C_T 4
COMPENSATION 1
FEEDBACK 2
–
2.5V
+
I_SENSE 3
REFERENCE ENABLE
5V REF
MAIN BIAS
UV
LOCKOUT
REFERENCE PULLDOWN
OUTPUT
PULLDOWN
OSCILLATOR
5.6V
1mA
1V
2R
–
R
+
S
R
BLANKING
18V
+
1.5V
–
8 V_REF
7 V_CC
6 OUTPUT
5 GND
LT1246 • BD01
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