CS1610 Datasheet, PDF (9/17 Pages) Cirrus Logic – TRIAC Dimmable LED Driver IC

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.
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 an
external resistor, R_IPK, on pin IPK, which is sampled
periodically by an ADC. Maximum power output is proportional
to I_PK(code). See Equation 1:
P_{in, max} = ^----^---^I--^P---^K----^--^B---^S----^T---^---^--₂---^V----^r--^m----^s--^,---^t-^y---^p---ⁱ--^c--^a---^l--^
[Eq. 1]
where,
 = correction term = 0.55
V_{rms, typical} = nominal operating input RMS voltage
I_PK(BST) = I_PK(code) x 4.1mA
The external resistor, R_IPK, is calculated using the peak
current code, I_PK(code). See Equation 2:
R_IPK = _I-4-P--0--_K-0--_--0_c---_o^0---_d^0---_e-0--_
[Eq. 2]
5.4.2 Output BSTOUT Sense & Input IAC Sense
A current proportional to the boost output voltage, V_BST, is
supplied to the IC on pin BSTOUT and is used as a feedback
control signal. The ADC is used to measure the magnitude of
the I_BSTOUT current through resistor R_BST. The magnitude of
the I_BSTOUT current is then compared to an internal reference
current (I_ref) of 133A.
IB S TOUT
VB S T
R8
RB S T
R9
CS1610 /11/12/13
16 ^{BSTOUT 15k}
Iref
ADC
12
Figure 9. BSTOUT Input Pin Model
Resistor R_BST sets the feedback current at the nominal boost
output voltage. For the CS1611/13, R_BST is calculated as
shown in Equation 3:
R_BST = ^V---_I--^Br--_e-^S--_f--^T- = ₁--⁴--₃--⁰-₃--⁰--_--^V--_A--  3M
[Eq. 3]
where,
V_BST = Nominal boost output voltage
I_ref = Internal reference current
For 120 VAC line voltage applications (CS1610/12), nominal
boost output voltage, V_BST, is 200V, and resistor R_BST is 1.5 M.
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.
V rec t
R3
IA C
RIA C
R4
CS1610 /11/12/13
2 ^{IAC 15k}
Iref
ADC
12
Figure 10. IAC Input Pin Model
Resistor R_IAC sets the I_AC current and is defined in Equation 4:
R_IAC = R_BST
[Eq. 4]
For optimal performance, R_IAC and R_BST should use 1% or
better resistors for best V_BST voltage accuracy.
5.4.3 Boost Auxiliary Winding
The boost auxiliary winding is used for zero-current detection
(ZCD). The voltage on the auxiliary winding is sensed through
the BSTAUX pin of the IC. It is also used to deliver current
during steady-state operation, as mentioned in section 5.2
Startup Circuit on page 8.
DS929F5
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