UCC28810_15 Datasheet, PDF (7/34 Pages) Texas Instruments – LED LIGHTING POWER CONTROLLER

UCC28810
UCC28811
www.ti.com.............................................................................................................................................................................................. SLUS865 – OCTOBER 2008
BLOCK DESCRIPTION
BLOCK DIAGRAM
2.7/2.5 V
+ ^OVP
V_REF
Init. Bias
V_REF and
Bias Reg
UVLO
+
0.67/0.57 V
0.23/0.15 V
+ ^Enable
VSENSE 1
2.5 V
g_M VEA
+
EAOUT 2
x
xK
Current
+
Reference
Generator
VINS 3
2.3 V
Zero Energy
+ ^Detect
ISENSE 4
40 kW
5 pF
75 mV
OVP
V_REF
GOOD
Current Sense Comparator
RQ
SQ
REF
Restart Timer
+ 1.7/1.4 V
8 VDD
7 GDRV
6 GND
5 TZE
UDG-08130
UVLO and Reference Circuit
This circuitry generates a precision reference voltage used to obtain a tightly controlled UVLO threshold. In
addition to generating a 2.5-V reference for the noninverting terminal of the g_M amplifier, it generates the
reference voltages for OVP, enable, zero energy detect and the current reference generator circuits. An internal
rail of 7.5 V is also generated to drive all the internal circuitry.
Error Amplifier
The voltage error amplifier in the UCC2881x is a transconductance amplifier with a typical transconductance
value of 90 µS. The advantage in using a transconductance amplifier is that the inverting input of the amplifier is
solely determined by the external resistive-divider from the output voltage and not the transient behavior of the
amplifier itself. This allows the VSENSE pin to be used for sensing over voltage conditions.
The sink and source capability of the error amplifier is approximately 10 µA during normal operation of the
amplifier. But when the VSENSE pin voltage is beyond the normal operating conditions (V_VSENSE >1.05 × V_REF,
V_VSENSE < 0.88 × V_REF), additional circuitry to enhance the slew-rate of the amplifier is activated. Enhanced
slew-rate of the compensation capacitor results in a faster start-up and transient response. This prevents the
output voltage from drifting too high or too low, which can happen if the compensation capacitor were to be
driven by the normal drive current of 10-µA. When VSENSE rises above the normal range, the enhanced sink
current capability increases to 1 mA, nominal. When VSENSE falls below the normal range, the UCC28810 can
source more than 1 mA and the UCC28811 sources approximately 300 µA. The limited source current in the
UCC28811 helps to gradually increase the error voltage on the EAOUT pin preventing a step increase in line
current. The actual rate of increase of the voltage on the EAOUT pin is dependent on the compensation network
externally connected to the EAOUT pin.
Copyright © 2008, Texas Instruments Incorporated
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