ssl1523 NXP Semiconductors, ssl1523 Datasheet - Page 5

• Manufacturer Part Number: ssl1523
• Description: Smps Ics For Mains Led Drivers
• Manufacturer: NXP Semiconductors

NXP Semiconductors
8. Functional description
SSL152x_1
Product data sheet
8.1 Start-up and undervoltage lockout
8.2 Oscillator
8.3 Duty factor control
The SSL152x family is the heart of a compact flyback converter, with the IC placed at the
primary side. The auxiliary winding of the transformer can be used for indirect feedback to
control the isolated output. This additional winding also powers the IC. A more accurate
control of the output voltage and/or current can be implemented with an additional
secondary sensing circuit and opto coupler feedback.
The SSL152x family uses voltage mode control. The switching frequency is determined by
the maximum transformer demagnetizing time and the time of the oscillator. In the first
case, the converter operates in the Self Oscillating Power Supply (SOPS) mode. In the
latter case, it operates at a constant frequency, which can be adjusted with external
components R
secondary ringing. This can use constant power or constant current mode to drive LEDs.
The valley switching principle minimizes capacitive switch-on losses.
Initially, the IC is self-supplying from the rectified mains voltage. The IC starts switching as
soon as the voltage on pin V
the auxiliary winding of the transformer as soon as V
from the line is stopped for high efficiency operation.
When for some reason the auxiliary supply is not sufficient, the high-voltage supply also
supplies the IC. As soon as the voltage on pin V
stops switching and restarts from the rectified mains voltage.
The frequency of the oscillator is set by the external resistor and capacitor on pin RC. The
external capacitor is charged rapidly to the V
stroke, it discharges to the V
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 systems with a linear sawtooth oscillator. Stable operation
at low duty factors is easily realized. For high efficiency, the frequency is reduced as soon
as the duty factor drops below a certain value. This is accomplished by increasing the
oscillator charge time.
To ensure that the capacitor can be charged within the charge time, the value of the
oscillator capacitor should be limited to approximately 1 nF.
The duty factor is controlled by the internal regulation voltage and the oscillator signal on
pin RC. The internal regulation voltage is equal to the external regulation voltage (minus
2.5 V) multiplied by the gain of the error amplifier (typically 20 dB).
RC
and C
Rev. 01 — 15 September 2008
RC
. Furthermore, a primary stroke is started only in a valley of the
CC
RC(min)
passes the V
level. Because the discharge is exponential, the
RC(max)
CC(startup)
CC
drops below the V
level and, starting from a new primary
CC
SMPS ICs for low power systems
level. The supply is taken over by
is high enough and the supply
CC(stop)
SSL152x
© NXP B.V. 2008. All rights reserved.
level, the IC
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