SSL2101T NXP [NXP Semiconductors], SSL2101T Datasheet - Page 5

• Manufacturer Part Number: SSL2101T
• Description: SMPS IC for dimmable LED lighting
• Manufacturer: NXP [NXP Semiconductors]

NXP Semiconductors
8. Functional description
SSL2101_4
Product data sheet
8.1 Start-up and UnderVoltage LockOut (UVLO)
8.2 Oscillator
The SSL2101 is a LED driver IC that operates directly from the rectified mains.
The SSL2101 uses on-time mode control and frequency control to control the LED
brightness. The BRIGHTNESS and PWMLIMIT input of the IC can be used to control the
LED light output in combination with an external dimmer. The PWMLIMIT input can also
be used for Thermal Lumen Management (TLM) and for precision LED current control.
Initially, the IC is self-supplying from the rectified mains voltage. The IC starts switching as
soon as the voltage on pin V
by the auxiliary winding of the transformer as soon as V
from the line is stopped for high efficiency operation. Alternatively the IC can be supplied
via a bleeder resistor connected to a high voltage. Note however the maximum V
voltage rating of the IC.
If for some reason the auxiliary supply is not sufficient, the high-voltage supply can also
supply the IC. As soon as the voltage on pin V
stops switching and will restart from the rectified mains voltage, if the internal current
delivered is sufficient.
An internal oscillator inside the IC provides the timing for the switching converter logic.
The frequency of the oscillator is set by the external resistors and the capacitor on pin RC
and pin RC2. The external capacitor is charged rapidly to the V
from a new primary stroke, it discharges to the V
exponential, the relative sensitivity of the duty factor to the regulation voltage at low duty
factor is almost equal to the sensitivity at high duty factors. This results in a more constant
gain over the duty factor range compared to Phase Width Modulated (PWM) systems with
a linear sawtooth oscillator. Stable operation at low duty factors is easily realized. The
frequency of the converter when V
R equals the parallel resistance of both oscillator resistors. C is the capacitor connected at
the RC pin (pin 8).
The BRIGHTNESS input controls the frequency reduction mode.
oscillator switches over from an RC curve with R1 in parallel with R2 to R1 only. A low
BRIGHTNESS voltage will reduce the switching frequency.
RC
=
------ -
3.5
1
--------- - t
f
1
osc
–
ch
arg
Rev. 04 — 28 August 2009
e
CC
passes the V
BRIGHTNESS
CC(startup)
is high can be estimated using
CC
RC(min)
drops below the V
SMPS IC for dimmable LED lighting
level. The supply can be taken over
level. Because the discharge is
CC
is high enough and the supply
RC(max)
Figure 3
CC(UVLO)
level and, starting
SSL2101
© NXP B.V. 2009. All rights reserved.
shows that the
level, the IC
Equation
CC
5 of 22
(1)
1: