Cirrus Logic CS1613 - Manual (Page 9)

CS1610/11/12/13
DS929F6 9
5.4 Boost Stage
The high-voltage FET in the source-follower startup circuit is
source-switched by a variable current source on the SOURCE
pin to operate a boost circuit. Peak FET switching current is
set with an external resistor on pin IPK.
In No-Dimmer Mode, the boost stage begins operating when
the start threshold is reached during each rectified half line-cy-
cle and is disabled at the nominal boost output voltage. The
peak FET switching current determines the percentage of the
rectified input voltage conduction angle over which the boost
stage will operate. The control algorithm adjusts the peak FET
switching current to maximize the operating time of the boost
stage, thus improving the input power factor.
When operating in Leading-edge Dimmer Mode, the boost
stage ensures the hold current requirement of the dimmer is
met from the initiation of each half-line dimmer conduction
cycle until the peak of the rectified input voltage. Trailing-edge
Dimmer Mode boost stage ensures that the trailing-edge is
exposed at the correct time with the correct current.
5.4.1 Maximum Peak Current
The maximum boost inductor peak current is set using
external resistor R
IPK
on pin IPK, which is sampled
periodically by an ADC. Maximum power output is proportional
to peak current code I
PK(code)
. See Equation 1:
where,

= a correction term of 0.55
V
rms(typ)
= nominal operating input RMS voltage
I
PK(BST)
= peak current code I
PK(code)

4.1mA
Resistor R
IPK
is calculated using peak current code I
PK(code)
.
See Equation 2:
5.4.2 Output BSTOUT Sense & Input IAC Sense
A current proportional to boost output voltage V
BST
is supplied
to the IC on pin BSTOUT and is used as a feedback control
signal (see Figure 9). The ADC is used to measure the
magnitude of current I
BSTOUT
through resistor R
BST
. The
magnitude of current I
BSTOUT
is then compared to an internal
reference current I
ref
of 133 A.
Resistor R
BST
sets the feedback current at the nominal boost
output voltage. For the CS1611/13, resistor R
BST
is calculated
as shown in Equation 3:
where,
V
BST
= nominal boost output voltage
I
ref
= internal reference current
For 120VAC line voltage applications (CS1610/12), nominal
boost output voltage V
BST
is 200V, and resistor R
BST
is 1.5M .
By using digital loop compensation, the voltage feedback
signal does not require an external compensation network.
A current proportional to the AC input voltage is supplied to the
IC on pin IAC and is used by the boost control algorithm (see
Figure 10).
Resistor R
IAC
sets current I
AC
and is defined in Equation 4:
For optimal performance, resistors R
IAC
and R
BST
should use
1% or better resistors for best V
BST
voltage accuracy.
P
IN max  

I
PK BST  
V
rms typ  
  
2
------------------------------------------------------------- =
[Eq.1]
R
IPK
4M 
I
PK code  
-----------------------
= [Eq.2]
V
B S T
CS1610 /11/12/13
15k
ADC
R8
R
B S T
I
B S TOUT
R9
I
ref
16
BSTOUT
12
Figure 9. BSTOUT Input Pin Model
R
BST
V
BST
I
ref
--------------
400V
133 A
------------------ 3M  
= = [Eq.3]
R3
R
IA C
I
A C
IAC
V
rect
CS1610 /11/12/13
15k
ADC
R4
2
I
ref
12
Figure 10. IAC Input Pin Model
R
IAC
R
BST
= [Eq.4]