SQ9910A datasheet(7/10 Pages) ETC2

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SQ9910/SQ9910A

Equation:



max

min

)

(

Line









Where

capacity charge work period,

generally about 0.2~0.25

f : input frequency for full range (85~265

rms

)

max



should be set 10~15％ of

min

Line

And

CMIN = ILED*VLEDs*0.06/VIN^2

CMIN = 22 µF, a value 22µF/250V can be used

A passive PFC circuit at the input requires using two

series connected capacitors at the place of calculated

CMIN. Each of these identical capacitors should be

rated for ½ of the input voltage and have twice as

much capacitance

Enable

The SQ9910 can be turned off by pulling the PWM_D

pin to ground. When disabled, the SQ9910 draws

quiescent current of less than 1mA

Output Open Circuit Protection

When the buck topology is used, and the LED is

connected in series with the inductor, there is no need for

any protection against an open circuit condition in the

LED string. Open LED connection means no switching

and can be continuous.

DC/DC Low Voltage Applications

Buck Converter Operation

SQ9910 is an offline AC-DC solution for LED lighting

system. Due to its simplicity of buck topology when the

LED string voltage is needed to be lower than the input

supply voltage, this solution can be designed to meet

various non-isolation application including T-8, LED

lamps

The design procedure for a buck LED driver outlined in

the previous chapters can be applied to the low

voltage LED drivers as well. However, the designer

must keep in mind that the input voltage must be

maintained higher than 2 times the forward voltage

drop across the LEDs. This limitation is related to the

output current instability that may develop when

the SQ9910 buck converter operates at a duty

cycle greater than 0.5. This instability reveals itself

as an oscillation of the output current at a sub-

harmonic of the switching frequency.

Benefiting from SQ9910 inherited high voltage

feature, rectified DC high voltage (Vdc = Vac x

1.414) can be directly fed into power pin of it to

achieve high duty cycle, which is only limited by

Vo / Vin, to optimize design efficiency. This

solution can easily achieve above 90% efficiencyl.

However, if the duty cycle is configured to reach

above more than 50%, some instability called

sub-harmonics oscillation (SBO) will occur.

The best solution is to adopt the so-called

constant off-time operation as shown in Fig. 2.

The resistor (R2) is, connected to ground by

default, to set operating frequency. We can

alternatively connect this resistor to gate of

MOSFET to force SQ9910 to enter constant OFF

time mode which will decrease duty cycle from

50% by increase total period, Toff + Ton.

One of the major scenarios which might induce

SBO is that when SQ9910 is biased right after

passive PFC stage. The DC voltage rail, Vin, is

halved which will easily increase duty cycle up to

more than 50%

For

general

LED

lighting

application,

PFC

becomes a necessary factor in order to meet the

emergent international standard of solid state

lighting. The following example as shown in Fig. 2

can explain it in more details where Rsense is

connected between Pin 8, Rosc, and Pin 4, GATE

to set SQ9910 to operate in fixed-off time mode

Example :

VIN - Vac 110V with passive PFC

VOUT - consisting of 1W HB LED with nominal Vf

=3.3V.

Vin, after rectified and passing PFC stage, the

actual DC rail will become

110V * 1.414 / 2 = 77.7Vdc

The duty cycle, D = Vout / Vin, will reach above

50% when voltage drop of LED string, as the

Vout, is more than 77.7 / 2 = 38.8V. Another word,

if any string consisting of 38.8 / 3.3 = 12 LEDs in a

series, SBO will occur.

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