AS3658 datasheet(18/158 Pages) AMSCO

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AS3658

Data Sheet Confidential - Detail ed Descri ption-Power Management Functions

The DC/DC Step Up converter is a high efficiency current mode PWM regulator, which provides an output voltage

dependent on the maximum VDS voltage of the external transistor, and maximum load current selectable by the

external shunt resistor.

For Example:

5V,500mA @ 1.1Mhz

25V,50mA @ 1.1MHz

40V,20mA @ 550kHz

A constant switching frequency results in a low noise on supply and output voltage.

8.1.1 Feedback selection

For step up DCDC 1, the feedback is always DCDC_FB1.

For step up DCDC 2 following feedback selections are possible:

Stpup2_fb selects the type of feedback for the DCDC_step_up2 converter:

DCDC_CURR1, DCDC_CURR2, DCDC_CURR3 or DCDC_FB2 feedback (see Figure 5)

Setting stpup2_fb to 00b enables the feedback on DCDC_FB2, stpup2_fb to 01b enables feedback at pin

DCDC_CURR1, setting step_up_fb to 10b enables feedback at pin DCDC_CURR2 and setting step_up_fb to 11b

enables feedback at pin DCDC_CURR3. The Step-up converter is regulated such that the required current at the

feedback path can be supported.

Always choose the path with the higher voltage drop as feedback to guarantee adequate supply for the other,

unregulated path.

To protect the DCDC output voltage against overvoltage, if a LED string is broken, set stpup2_prot=1. In this mode the

output voltage will be limited by limiting the DCDC_FB voltage to 1.25V (select the external resistor network to adjust

this limitation voltage).

Cout

Output capacitor

2.2

µF

ceramic,

±20%

Inductor

µH

Use inductors with small Cparasitic

(<100pF) to get high efficiency

tMIN_ON

Minimum on time

130

MDC

Maximum duty cycle

Table 5. DC/DC Converter parameters

Symbol

Parameter

Min

Typ

Max

Unit

Note

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