LED6000 datasheet(19/49 Pages) STMICROELECTRONICS

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LED6000

Functional description

4.4

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 0.25 V 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 the high DC gain and low output

impedance.

The uncompensated error amplifier characteristics are summarized inTable 6.

In the 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 5.3 on page 27 for details on the compensation network

design).

In case of the light-load (i.e.: if the output current is lower than the half of the inductor current

ripple) the LED6000 enters the 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 the discontinuous

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

to charge the inductor up to the ISKIP level.

The ISKIP threshold is reduced with increasing the RILIM resistor value, as shown also in

Table 5 on page 8 and plotted in Figure 12, so allowing the LED6000 device work in the

continuous conduction mode also in case lower current LEDs are selected.

Table 6. Error amplifier characteristics

Parameters

Value

Low frequency gain (A0)100 dB

GBWP

23 MHz

Output voltage swing

0 to 3.5 V

Source/sink current capability

3.1 mA / 5 mA

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