ALED6000 datasheet(15/45 Pages) STMICROELECTRONICS

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The above consideration is crucial when short DIM pulses are expected in the final application. Once the external

power components and the compensation network are selected, a direct measurement to determine tRISE and

tFALL is necessary to certify the achieved dimming performance.

When DIM is forced above the VDIMH threshold after TDIMTO has elapsed, a new soft-start sequence is performed.

5.4

Error amplifier and light-load management

The error amplifier (E/A) provides the error signal to be compared with the sawtooth to perform the pulse width

modulation. Its non inverting input is internally connected to a 0.25V voltage reference and its inverting input (FB)

and output (COMP) are externally available for feedback and frequency compensation. In this device the error

amplifier is a voltage mode operational amplifier, therefore, with high DC gain and low output impedance.

The uncompensated error amplifier characteristics are summarized in Table 6. Error amplifier characteristics .

Table 6. Error amplifier characteristics

Parameters

Value

Low frequency gain (A0)

100 dB

GBWP

23 MHz

Output voltage swing

0 to 3.5V

Source/sink current capability

3.1 mA / 5 mA

In continuous conduction working mode (CCM), the transfer function of the power section has two poles due to

the LC filter and one zero due to the ESR of the output capacitor. Different kinds of compensation networks can

be used depending on the ESR value of the output capacitor.

If the zero introduced by the output capacitor helps to compensate the double pole of the LC filter, a

type II compensation network can be used. Otherwise, a type III compensation network must be used (see

Section 6.3 Compensation strategy for details on the compensation network design).

In case of light load (i.e. if the output current is lower than the half of the inductor current ripple) the ALED6000

enters pulse-skipping working mode. The HS MOS is kept off if the COMP level is below 200 mV (typ.); when

this bottom level is reached the integrated switch is turned on until the inductor current reaches ISKIP value. So,

in discontinuous conduction working mode (DCM), the HS MOS on-time is only related to the time necessary to

charge the inductor up to ISKIP level.

ISKIP threshold is reduced with increasing RILIM resistor value, as shown also in Table 5. Electrical characteristics

and plotted in Figure 12. ISKIP typical current and RILIM value , so allowing the ALED6000 to work in continuous

conduction mode also in case lower current LEDs is selected.

ALED6000

Error amplifier and light-load management

DS13018 - Rev 4

page 15/45

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