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參數(shù)資料
| 型號： | LTC3707IGN-SYNC |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A SWITCHING CONTROLLER, 360 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封裝： | 0.150 INCH, PLASTIC, SSOP-28 |
| 文件頁數(shù)： | 11/32頁 |
| 文件大?。?/td> | 403K |
| 代理商： | LTC3707IGN-SYNC |
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LTC3707-SYNC
19
3707sfa
the VOUT resistive divider to compensate for the current
comparator’s negative input bias current. The maximum
current owing out of each pair of SENSE pins is:
ISENSE+ + ISENSE– = (2.4V – VOUT)/24k
Since VOSENSE is servoed to the 0.8V reference voltage,
we can choose R1 in Figure 2 to have a maximum value
to absorb this current.
Rk
V
VV
MAX
OUT
124
08
24
()
.
.–
=
for VOUT < 2.4V
Regulating an output voltage of 1.8V, the maximum value
of R1 should be 32k. Note that for an output voltage above
2.4V, R1 has no maximum value necessary to absorb
the sense currents; however, R1 is still bounded by the
VOSENSE feedback current.
Soft-Start/Run Function
The RUN/SS1 and RUN/SS2 pins are multipurpose pins
that provide a soft-start function and a means to shut down
the IC. Soft-start reduces the input power source’s surge
currents by gradually increasing the controller’s current
limit (proportional to VITH). This pin can also be used for
power supply sequencing.
An internal 1.2μA current source charges up the CSS
capacitor. When the voltage on RUN/SS1 (RUN/SS2)
reaches 1.5V, the particular controller is permitted to
start operating. As the voltage on RUN/SS increases from
1.5V to 3.0V, the internal current limit is increased from
25mV/RSENSE to 75mV/RSENSE. The output current limit
ramps up slowly, taking an additional 1.25s/μF to reach full
current. The output current thus ramps up slowly, reducing
the starting surge current required from the input power
supply. If RUN/SS has been pulled all the way to ground
there is a delay before starting of approximately:
t
V
A
C
s F C
DELAY
SS
==
()
15
12
125
.
./
t
VV
A
C
s F C
IRAMP
SS
=
=
()
31 5
12
125
.
./
APPLICATIONS INFORMATION
By pulling both RUN/SS pins below 1V, the IC is put into
low current shutdown (IQ = 20μA). The RUN/SS pins
can be driven directly from logic as shown in Figure 7.
Diode D1 in Figure 7 reduces the start delay but allows
CSS to ramp up slowly providing the soft-start function.
Each RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pins also provide the ability to latch off the
controller(s) when an overcurrent condition is detected.
The RUN/SS capacitor, CSS, is used initially to turn on
and limit the inrush current. After the controller has been
started and been given adequate time to charge up the
output capacitor and provide full load current, the RUN/SS
capacitor is used for a short-circuit timer. If the regulator’s
output voltage falls to less than 70% of its nominal value
after CSS reaches 4.1V, CSS begins discharging on the as-
sumption that the output is in an overcurrent condition. If
the condition lasts for a long enough period as determined
by the size of the CSS and the specied discharge current,
the controller will be shut down until the RUN/SS pin volt-
age is recycled. If the overload occurs during start-up, the
time can be approximated by:
tLO1 ≈ [CSS(4.1 – 1.5 + 4.1 – 3.5)]/(1.2μA)
= 2.7 106 (CSS)
If the overload occurs after start-up the voltage on CSS will
begin discharging from the zener clamp voltage:
tLO2 ≈ [CSS (6 – 3.5)]/(1.2μA) = 2.1 106 (CSS)
Figure 7. RUN/SS Pin Interfacing
3.3V OR 5V
RUN/SS
VIN
INTVCC
RUN/SS
D1
CSS
RSS*
CSS
RSS*
3707 F07
(a)
(b)
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
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