Datasheet

4
UC2855A/B
UC3855A/B
ELECTRICAL CHARACTERISTICS:
Unless otherwise specified: VCC = 18V, RVS = 23k, CT = 470pF, CI = 150pF, VRMS
= 1.5V, IAC = 100µA, I
SENSE
= 0V, CAO = 4V, VAOUT= 3.5V, VSENSE = 3V. –40°C to 85°C (UC2855A/B), 0°Cto70°C
(UC3855A/B).
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
Gate Driver Output
Output High Voltage l
OUT
= −200mA, VCC = 15V 12 12.8 V
Output Low Voltage l
OUT
= 200mA 1 2.2 V
Output Low Voltage l
OUT
= 10mA 300 500 mV
Output Low (UVLO) lOUT = 50mA, VCC = 0V 0.9 1.5 V
Output RISE/FALL Time C
LOAD = 1nF 35 ns
Output Peak Current C
LOAD = 10nF 0.5 1.5 A
ZVT
ZVS Threshold 2.3 2.6 2.9 V
Input Bias Current V = 2.5V, V
CT
=0 6 20 µA
Propagation Delay Measured at ZVTOUT 100 ns
Maximum Pulse Width 400 ns
Output High Voltage l
OUT
= −100mA, VCC = 15V 12 12.8 V
Output Low Voltage l
OUT
= 100mA 1 2.2 V
l
OUT
= 10mA 300 900 mV
Output Low (UVLO) l
OUT
= 50mA, V
CC
= 0V 0.9 1.5 V
Output RISE/FALL Time C
LOAD
= 1nF 35 ns
Output Peak Current C
LOAD
= 10nF 0.25 0.75 A
Current Synthesizer
I
ON to CS Offset VION = 0V 30 50 mV
Cl Discharge Current IAC = 50µA 105 118 140 µA
IAC = 500µA5µA
IAC Offset Voltage 0.3 0.65 1.1 V
ION Buffer Slew Rate 10 V/µs
ION Input Bias Current V
ION
=2V 2 15 µA
RVS Output Voltage 23k from RVS to GND 2.87 3 3.13 V
Note 1: Gain constant (K) =
IAC V A V
VIMO
OUT
RMS
•
•
(–.)
()
15
2
at V
RMS
= 1.5V, VA
OUT
= 5.5V.
PIN DESCRIPTIONS
CA This is the inverting input to the current amplifier.
Connect the required compensation components be
-
tween this pin and CAOUT. The common mode operating
range for this input is between −0.3V and 5V.
CAO
: This is the output of the wide bandwidth current
amplifier and one of the inputs to the PWM duty cycle
comparator. The output signal generated by this amplifier
commands the PWM to force the correct input current.
The output can swing from 0.1V to 7.5V.
CI
: The level shifted current sense signal is impressed
upon a capacitor connected between this pin and GND.
The buffered current sense transformer signal charges
the capacitor when the boost switch is on. When the
switch is off, the current synthesizer discharges the ca
-
pacitor at a rate proportional to the dI/dt of the boost in
-
ductor current. In this way, the discharge current is
approximately equal to
3
4
V
RRVS
IAC
–
.
Discharging the CI capacitor in this fashion, a “recon
-
structed” version of the inductor current is generated us
-
ing only one current sense transformer.