CS1600_1007 Datasheet, PDF (7/18 Pages) Cirrus Logic – LOW-cost PFC Controller for Electronic Ballasts

CS1600
4. INTRODUCTION
The CS1600 is a digitally controlled Power Factor Correction
(PFC) controller that operates in the Variable Frequency
Discontinuous Conduction Mode (VF - DCM). The CS1600
uses a proprietary digital algorithm to optimize control of the
power switch to deliver highly efficient performance for
electronic ballast applications. With this control scheme, the
total number of external components needed is minimized in
comparison to conventional control techniques, thus reducing
the overall system cost.
Digital control is achieved by constantly monitoring two voltages
– the PFC output voltage (V_link) at pin FB and the rectified AC line
voltage (V_rect) at pin IAC. This is done by measuring the currents
that flow into the respective pins. These currents are then fed to
the inputs of two analog-to-digital converters (ADCs) and are
compared against an internal target current, I_ref.
The digital outputs of the two ADCs are then processed in a
control algorithm which determines the behavior of the
CS1600 during start-up, normal operation, and under fault
conditions such as brownout, overvoltage, overcurrent,
overpower, and over-temperature. Details of operation during
these conditions are discussed in later sections of this
document.
Some of the key features of the CS1600 are as follows:
• Discontinuous Conduction Mode with Continuously
Variable Switching Frequency
The PFC switching frequency is varied every switching
cycle. This allows for a spread spectrum which minimizes
the conducted EMI peaks at any given frequency, thereby
minimizing the size and cost of the EMI filter required at
the front-end.
During start-up, the control algorithm limits the maximum
ON time and adjusts the frequency to avoid inductor sat-
uration and provides a near-trapezoidal envelope for the
input current during every half cycle. During normal oper-
ation, as the line voltage changes over half of a line cycle,
the frequency varies approximately 2:1 as shown in
Figure 7 below.
120
Switching Frequency (% of Max)
100
80
60
Line Voltage (% of Max)
40
20
0
0
45
90
135
180
Rectified Line Voltage Phase (Deg.)
Figure 7. Switching Frequency vs. Phase Angle
Maximum power transfer occurs at the peak of the AC line
voltage, at which time, the frequency reaches its maxi-
mum value. Switching losses are minimized during peri-
ods of low power transfer by switching at lower
frequencies near the zero-crossing of the AC line.
This switching frequency profile helps reduce total BOM
cost through savings in the size of the boost inductor and
the EMI filter components, while at the same time, im-
proving overall system efficiency.
• Integrated Feedback Control
No external feedback compensation components are re-
quired for the CS1600. The internal digital control engine
self-compensates the feedback error signal using an
adaptive control algorithm.
• Protection Features
The CS1600 provides various protection features such as
undervoltage, overcurrent, overpower, open and short
circuit protection and brownout. It also provides the user
with the option of using the STBY pin to disable switching
of the device.
DS904A7
7