LM5017 datasheet(13/33 Pages) TI1

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VOUT

RFB2

VCC

RFB1

To FB

RFB1 x RFB2

RFB1 + RFB2

tS = C1 x (R2 +

)

LM5017

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SNVS783I – JANUARY 2012 – REVISED OCTOBER 2015

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To achieve the desired soft-start, the following design guidance is recommended:

(1) R2 is selected so that VFB is higher than 1.225 V for a VCC of 4.5 V, but is lower than 5 V when VCC is 8.55 V.

If an external VCC is used, VFB should not exceed 5 V at maximum VCC.

(2) C1 is selected to achieve the desired start-up time that can be determined from Equation 9.

(9)

(3) R1 is used to maintain the node B voltage at zero after the soft-start is finished. A value larger than the

feedback resistor divider is preferred. Note that the effect of R1 is ignored in the previous equations.

Based on the schematic shown in Figure 13, selecting C1 = 1 uF, R2 = 1 kΩ, R1 = 30 kΩ results in a soft-start

time of about 2 ms.

Figure 12. Soft-Start Circuit

7.4 Device Functional Modes

Table 2. UVLO Modes

UVLO

VCC Regulator

MODE

DESCRIPTION

< 0.66 V

Disabled

Shutdown

VCC regulator disabled.

Switching disabled.

0.66 V – 1.225 V

Enabled

Standby

VCC regulator enabled

Switching disabled.

> 1.225 V

VCC < 4.5 V

Standby

VCC regulator enabled.

Switching disabled.

VCC > 4.5 V

Operating

VCC enabled.

Switching enabled.

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