TPS51220A_16 Datasheet, PDF (21/51 Pages) Texas Instruments – Fixed Frequency, 99% Duty Cycle Peak Current Mode Notebook System Power Controller

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TPS51220A
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_EX / I_EN12, which means the time from ENx = 1 V to ENx = 2 V. The recommend
capacitance is more than 2.2 nF.
1) Internal
Soft-start
EN1
Vout1
2) External
Soft-start
EN1
200ms
960ms
EN1>1V
EN1<2V
Vout1
External
Soft-start
time
Figure 40. Enable and Soft-start Timing
Table 1. Enable Logic States
EN
GND
Hi
Hi
Hi
Hi
EN1
Don’t Care
Lo
Hi
Lo
Hi
EN2
Don’t Care
Lo
Lo
Hi
Hi
VREG3
ON
ON
ON
ON
ON
VREF2
Off
ON
ON
ON
ON
VREG5
Off
ON
ON
ON
ON
CH1
Off
Off
ON
Off
ON
CH2
Off
Off
Off
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.
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