Datasheet

200ms

960ms

Vout1

EN1

Vout1

External

Soft-start

time

EN1>1V

EN1<2V

1)Internal

Soft-start

2)External

Soft-start

TPS51220A

www.ti.com

SLUS897E –DECEMBER 2008–REVISED JANUARY 2013

DETAILED DESCRIPTION

ENABLE AND SOFT START

When EN is Low, the TPS51220A is in the shutdown state. Only the 3.3-V LDO stays alive, and consumes 7 μA

(typically). When EN becomes High, the TPS51220A is in the standby state. The 2-V reference and the 5-V LDO

become enabled, and consume about 80 μA with no load condition, and are ready to turn on SMPS channels.

Each SMPS channel is turned on when ENx becomes High. After ENx is set to high, the TPS51220A begins the

softstart sequence, and ramps up the output voltage from zero to the target voltage in 0.96 ms. However, if a

slower soft-start is required, an external capacitor can be tied from the ENx pin to GND. In this case, the

TPS51220A charges the external capacitor with the integrated 2-μA current source. An approximate external

soft-start time would be t

EX-SS

= C

/ I

EN12

, which means the time from ENx = 1 V to ENx = 2 V. The recommend

capacitance is more than 2.2 nF.

Figure 40. Enable and Soft-start Timing

Table 1. Enable Logic States

EN EN1 EN2 VREG3 VREF2 VREG5 CH1 CH2

GND Don’t Care Don’t Care ON Off Off Off Off

Hi Lo Lo ON ON ON Off Off

Hi Hi Lo ON ON ON ON Off

Hi Lo Hi ON ON ON Off ON

Hi Hi Hi ON ON ON ON ON

PRE-BIASED START-UP

The TPS51220A supports a pre-biased start up by preventing negative inductor current during soft-start when

the output capacitor holds some charge. The initial DRVH signal waits until the voltage feedback signal becomes

greater than the internal reference ramping up by the soft-start function. After that, the start-up occurs in the

same way the soft-start condition fully discharges, regardless of the SKIPSELx selection.

Product Folder Links :TPS51220A