Cirrus Logic CS1631 - Manual (Page 4)

CS1630/31
4 DS954F3
1. INTRODUCTION
Figure 1. CS1630/31 Block Diagram
A typical schematic using the CS1630/31 IC is shown on the
front page.
Startup current is provided from a patent-pending, external
high-voltage source-follower network. In addition to providing
startup current, this unique topology is integral in providing
compatibility with digital dimmers by ensuring V
DD
power is
always available to the IC. During steady-state operation, an
auxiliary winding on the boost inductor back-biases the
source-follower circuit and provides steady-state operating
current to the IC to improve system efficiency.
The rectified input voltage is sensed as a current into pin IAC
and is used to control the adaptive dimmer compatibility
algorithm and extract the phase of the input voltage for output
dimming control. During steady-state operation, the external
high-voltage, source-follower circuit is source-switched in
critical conduction mode (CRM) to boost the input voltage.
This allows the boost stage to maintain good power factor,
provide dimmer compatibility, reduce bulk capacitor ripple
current, and provide a regulated input voltage to the second
stage.
The current into the boost output voltage sense pin BSTOUT
senses the output voltage of the CRM boost front-end.
The quasi-resonant second stage is implemented with peak-
current mode primary-side control, which eliminates the need
for additional components to provide feedback from the
secondary and reduces system cost and complexity.
Voltage across an external user-selected resistor is sensed
through pin FBSENSE to control the peak current through the
second stage inductor. Leading-edge and trailing-edge
blanking on pin FBSENSE prevents false triggering.
Pin FBAUX is used to sense the second stage inductor
demagnetization to ensure quasi-resonant switching of the
output stage.
An internal current source is adjusted by a feedback loop to
regulate a constant reference voltage on pin eOTP for external
negative temperature coefficient (NTC) thermistor
measurements. An external NTC is connected to pin eOTP to
provide thermal protection of the system and LED temperature
compensation. The output current of the system is steadily
reduced when the system temperature exceeds a
programmable temperature set point. If the temperature
reaches a designated high set point, the IC is shutdown and
stops switching.
V
Z
POR +
-
Voltage
Regulator
14
VDD
11
FBSENSE
+
-
15
FBAUX
+
-
13
GD
2
IAC
DAC
+
-
Peak
Control
Flyback ZCD
+
-
Output Open
12
GND
OLP
+
-
16
BSTOUT
OCP
+
-
1
BSTAUX
Boost ZCD
3
CLAMP
V
S T(th )
V
S TP(th )
V
OCP (th )
V
FB ZCD(th)
V
OV P (th )
V
OLP (th)
V
FB ZCD(th)
V
P k_Max(th)
10
4
SGND
5
SOURCE
+
-
+
-
VDD
I
CONNE CT
V
CONNE CT(th)
V
S OURCE(th )
7 SCL
6
SDA
8
NC
9
SYNC
eOTP
15k
ADC MUX
15k
VDD
I
ref
t
FB ZCD
I
CLA MP
t
B S TZCD
I
S OURCE
Blank 3