8
Soft-Start Circuit
Soft-Start into Sleep States (S3, S4/S5)
The 5VSB POR function initiates the soft-start sequence. An
internal 10
μ
A current source charges an external capacitor
to 5V. The error amplifiers reference inputs are clamped to a
level proportional to the SS (Soft-Start) pin voltage. As the
SS pin voltage slews from about 1.25V to 2.5V, the input
clamp allows a rapid and controlled output voltage rise.
Figure 9 shows the soft-start sequence for the typical
application start-up in a sleep state with all output voltages
enabled. At time T0 5V
SB
(bias) is applied to the circuit. At
time T1, 5V
SB
surpasses POR level, and an internal fast
charge circuit quickly raises the SS capacitor voltage to
approximately 1V. At this point, the 10
μ
A current source
continues the charging up to T2, where a voltage of 1.25V
(typically) is reached and an internal clamp limits further
charging. Clamping of the soft-start voltage (T2 to T3
interval) should only be noticed with capacitors smaller than
0.1
μ
F; soft-start capacitors of 0.1
μ
F and above should
present a soft-start ramp void of this plateau. At time T3,
3ms (typically) past the 5V
SB
POR (T1), the memory output
voltage selection is latched in and the charging of the soft-
start capacitor resumes, using the 10
μ
A current source. At
this point, the error amplifiers’ reference inputs are starting
their transitions, causing the output voltages to ramp up
proportionally. The ramping continues until time T4 when all
the voltages reach the set value. At time T5, when the soft-
start capacitor value reaches approximately 2.8V, the under-
voltage monitoring circuits are activated and the soft-start
capacitor is quickly discharged down to the value attained at
time T2 (approximately 1.25V).
FIGURE 6. 3V
DUAL
AND 5V
DUAL
TIMING DIAGRAM FOR
EN3VDL = 0, EN5VDL = 1
FIGURE 7. 3V
DUAL
AND 5V
DUAL
TIMING DIAGRAM FOR
EN3VDL = 0, EN5VDL = 0
FIGURE 8. 2.5/3.3V
MEM
TIMING DIAGRAM
5VSB
12V
S3
S5
5VDLSB
DLA
3V3DLSB
3V3DL
5VDL
5VSB
12V
S3
S5
5VDLSB
DLA
3V3DLSB
3V3DL
5VDL
5VSB
12V
S3
S5
DRV2
DEVICE
VSEN2
INTVSEN2
FIGURE 9. SOFT-START INTERVAL IN A SLEEP STATE (ALL
OUTPUTS ENABLED)
0V
0V
TIME
SOFT-START
(1V/DIV)
OUTPUT
VOLTAGES
(1V/DIV)
V
OUT1
(3.3V
DUAL
)
V
OUT2
(2.5V
MEM
)
V
OUT3
(5V
DUAL
)
T1 T2T3
T0
5VSB
(1V/DIV)
UV DETECT ENABLE
(LOGIC LEVEL)
T5
T4
HIP6501A