CS1610/11/12/13
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.
VBST
R8
R9
IBSTOUT
RBST
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.
CS1610 /11/12/13
Iref
15k
BSTOUT
16
12
ADC
Figure 9. BSTOUT Input Pin Model
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.
Resistor RBST sets the feedback current at the nominal boost
output voltage. For the CS1611/13, resistor RBST is calculated
as shown in Equation 3:
VBST
-------------
Iref
400V
-----------------
3M
RBST
=
=
[Eq.3]
133A
5.4.1 Maximum Peak Current
where,
The maximum boost inductor peak current is set using
external resistor RIPK on pin IPK, which is sampled
periodically by an ADC. Maximum power output is proportional
to peak current code IPK(code). See Equation 1:
VBST = nominal boost output voltage
Iref = internal reference current
For 120VAC line voltage applications (CS1610/12), nominal
boost output voltage VBST is 200V, and resistor RBST is 1.5M.
By using digital loop compensation, the voltage feedback
signal does not require an external compensation network.
IPKBST Vrmstyp
[Eq.1]
------------------------------------------------------------
=
PINmax
2
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).
where,
= a correction term of 0.55
Vrms(typ) = nominal operating input RMS voltage
Vrect
IPK(BST) = peak current code IPK(code) 4.1mA
Resistor RIPK is calculated using peak current code IPK(code)
.
R3
See Equation 2:
IAC
RIA C
4M
IPKcode
R4
CS1610 /11/12/13
-----------------------
=
RIPK
[Eq.2]
Iref
15k
IAC
2
ADC
5.4.2 Output BSTOUT Sense & Input IAC Sense
A current proportional to boost output voltage VBST 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 IBSTOUT through resistor RBST. The
magnitude of current IBSTOUT is then compared to an internal
reference current Iref of 133A.
12
Figure 10. IAC Input Pin Model
Resistor RIAC sets current IAC and is defined in Equation 4:
RIAC = RBST
[Eq.4]
For optimal performance, resistors RIAC and RBST should use
1% or better resistors for best VBST voltage accuracy.
DS929F6
9
CIRRUS [ CIRRUS LOGIC ]
