SSL2129AT datasheet(8/21 Pages) NXP

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SSL2129AT

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Product data sheet

Rev. 2 — 23 October 2012

8 of 21

NXP Semiconductors

SSL2129AT

Dimmable LED controller IC

In buck mode, the rate of current rise in the coil during the on-phase is proportional to the

difference between input voltage and output voltage. Therefore, the peak current cannot

be reached before ton(high) and as a result the average output current to the LEDs is

reduced.

8.6 ton control

The ton(high) can be lowered by connecting a capacitor to the TONMOD pin. The external

capacitor is charged during the primary stroke with ITONMOD. If the VTONMOD level is

reached before the ton(high) time, the switch is turned off and the secondary stroke starts.

When a capacitor is not connected to the pin, VTONMOD is reached quickly. Shorter than

the minimum limit of 1

s. In this case or when the TONMOD pin is grounded, the internal

time constant, ton(high) determines the maximum on-time. This function is used to control

the converter operation over the mains cycle which enables the design of a mains

dimmable driver.

8.7 VCC supply

The SSL2129AT can be supplied using three methods:

• Under normal operation, the voltage swing on the DRAIN pin is rectified using

external components providing current towards the VCC pin

• At start-up, there is an internal current source connected to the HV pin. The current

source provides internal power until an external current on the VCC pin provides the

supply.

• Using an auxiliary winding, the voltage can be rectified and connected to the VCC pin.

The IC starts up when the voltage at the VCC pin is higher than VCC(startup). The IC locks

out (stops switching) when the voltage at the VCC pin is lower than VCC(stop). The

hysteresis between the start and stop levels allows the IC to be supplied by a buffer

capacitor until the external supply is settled. The SSL2129AT has an internal VCC clamp,

which is an internal active Zener (or shunt regulator). This internal active Zener limits the

voltage on the supply VCC pin to the maximum value of VCC. If the maximum current of

the supply minus the current consumption of the IC (determined by the load on the gate

drivers), is lower than the maximum value of IDD no external Zener diode is needed in the

supply circuit.

8.7.1 VCC regulator

During supply dips, the input voltage can drop too low to supply the required IC current.

Under these conditions, if the VCC voltage drops lower than VCC(swon)reg level, a second

regulator is started. Its function is to fill in the required supply current which the external

supply does not deliver. It prevents the IC going into UVLO. When the VCC voltage is

higher than VCC(swon)reg level, the regulator is turned off.

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