CS1610/11/12/13

DS929F3

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 an

periodically by an ADC. Maximum power output is proportional

where,

5.4.2

Output BSTOUT Sense and Input IAC

Sense

supplied to the IC on pin BSTOUT and is used as a feedback

control signal. The ADC is used to measure the magnitude of

Figure 9. BSTOUT Input Pin Model

shown in Equation 3:

where,

For 120VAC line voltage applications (CS1610/12), nominal

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.

Figure 10. IAC Input Pin Model

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.

,

PK BST



,





2

--------------------------------------------------------------------

=

[Eq.1]

4000000



-----------------------

=

[Eq.2]

CS1610 /11/12/13

15k

ADC

R8

R9

16

BSTOUT

12

--------------

400V

133

A

------------------

3M





==

[Eq.3]

R3

I

AC

CS1610 /11/12/13

15k

ADC

R4

2

12

=

[Eq.4]