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UC2845A Datasheet

HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

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UC284XA
UC384XA
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
NOT FOR NEW DESIGN
1 FEATURES
Figure 1. Package
TRIMMED OSCILLATOR DISCHARGE
CURRENT
CURRENT MODE OPERATION TO 500kHz
AUTOMATIC FEED FORWARD
COMPENSATION
DIP-8
SO-8
LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
Table 1. Order Codes
INTERNALLY TRIMMED REFERENCE WITH
Part Number
Package
UNDERVOLTAGE LOCKOUT
HIGH CURRENT TOTEM POLE OUTPUT
t(s)UNDERVOLTAGE LOCKOUT WITH
HYSTERESIS
cLOW START-UP CURRENT (< 0.5mA)
uDOUBLE PULSE SUPPRESSION
rod2 DESCRIPTION
PThe UC384xA family of control ICs provides the
necessary features to implement off-line or DC to
teDC fixed frequency current mode control schemes
lewith a minimal external parts count. Internally im-
oplemented circuits include a trimmed oscillator for
sprecise DUTY CYCLE CONTROL under voltage
blockout featuring start-up current less than 0.5mA,
Oa precision reference trimmed for accuracy at the
-error amp input, logic to insure latched operation,
)a PWM comparator which also provides current
t(slimit control, and a totem pole output stage de-
csigned to source or sink high peak current. The
uoutput stage, suitable for driving N-Channel MOS-
dFETs, is low in the off-state.
UC2842AD1; UC3842AD1;
UC2843AD1; UC3843AD1;
UC2844AD1; UC3844AD1;
UC2845AD1; UC3845AD1
UC2842AN; UC3842AN;
UC2843AN; UC3843AN;
UC2844AN; UC3844AN;
UC2845AN; UC3845AN
SO-8
DIP-8
Differences between members of this family are
the under-voltage lockout thresholds and maxi-
mum duty cycle ranges. The UC3842A and
UC3844A have UVLO thresholds of 16V (on) and
10V (off), ideally suited off-line applications The
corresponding thresholds for the UC3843A and
UC3845A are 8.5 V and 7.9V. The UC3842A and
UC3843A can operate to duty cycles approaching
100%. A range of the zero to < 50 % is obtained by
the UC3844A and UC3845A by the addition of an
internal toggle flip flop which blanks the output off
every other clock cycle.
ProFigure 2. Block Diagram (toggle flip flop used only in UC3844A and UC3845A)
te 7
Vi
le 5
Obso GROUND
34V
UVLO
2.50V
S/R
5V
REF
VREF GOOD
INTERNAL
BIAS
8 VREF
5V 50mA
LOGIC
6
OUTPUT
4
RT/CT
OSC
T
VFB
COMP
CURRENT
SENSE
2
1
3
ERROR AMP.
+
-
2R
R
S
R
1V
CURRENT
SENSE
COMPARATOR
PWM
LATCH
D95IN331
May 2004
REV. 5
1/16


STMicroelectronics Electronic Components Datasheet

UC2845A Datasheet

HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

No Preview Available !

UC384XA - UC284XA
Table 2. Absolute Maximum Ratings
Symbol
Parameter
Value
Unit
Vi Supply Voltage (low impedance source)
30 V
Vi Supply Voltage (Ii < 30mA)
Self Limiting
IO Output Current
±1 A
EO Output Energy (capacitive load)
5 µJ
Analog Inputs (pins 2, 3)
– 0.3 to 5.5
V
Error Amplifier Output Sink Current
10 mA
Ptot Power Dissipation at Tamb 25 °C (DIP-8)
1.25
W
Ptot Power Dissipation at Tamb 25 °C (SO-8)
800 mW
Tstg Storage Temperature Range
– 65 to 150
°C
TJ Junction Operating Temperature
– 40 to 150
°C
TL Lead Temperature (soldering 10s)
300 °C
* All voltages are with respect to pin 5, all currents are positive into the specified terminal.
)Figure 3. DIP-8/SO-8 Pin Connection (Top view)
duct(sCOMP
roVFB
te PISENSE
oleRT/CT
18
27
36
45
D95IN332
VREF
Vi
OUTPUT
GROUND
) - ObsTable 3. Pin Description
t(sN° Pin
Function
uc1 COMP This pin is the Error Amplifier output and is made available for loop compensation.
rod2 VFB This is the inverting input of the Error Amplifier. It is normally connected to the switching power
supply output through a resistor divider.
te P3 ISENSE A voltage proportional to inductor current is connected to this input. The PWM uses this
information to terminate the output switch conduction.
ole4 RT/CT The oscillator frequency and maximum Output duty cycle are programmed by connecting
s resistor RT to Vref and cpacitor CT to ground. Operation to 500kHz is possible.
Ob5 GROUND This pin is the combined control circuitry and power ground.
6 OUTPUT This output directly drives the gate of a power MOSFET. Peak currents up to 1A are sourced
and sunk by this pin.
7 VCC This pin is the positive supply of the control IC.
8 Vref This is the reference output. It provides charging current for capacitor CT through resistor RT.
2/16


Part Number UC2845A
Description HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
Maker ST Microelectronics
Total Page 16 Pages
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