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参数资料
| 型号： | LTC1929IG-PG |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A DUAL SWITCHING CONTROLLER, 310 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封装： | 0.209 INCH, PLASTIC, SSOP-28 |
| 文件页数： | 3/28页 |
| 文件大小： | 310K |
| 代理商： | LTC1929IG-PG |
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11
LTC1929/LTC1929-PG
OPERATIO
U
(Refer to Functional Diagram)
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both VOUT+ and VOUT– benefits regula-
tion in high current applications and/or applications hav-
ing electrical interconnection losses. The AMPMD pin
(LTC1929 only) allows selection of internal, precision feed-
back resistors for high common mode rejection differencing
applications, or direct access to the actual amplifier inputs
without these internal feedback resistors for other applica-
tions. The AMPMD pin is grounded to connect the internal
precision resistors in a unity-gain differencing application
(default for the LTC1929-PG), or tied to the INTVCC pin to
bypass the internal resistors and make the amplifier inputs
directly available. The amplifier is a unity-gain stable, 2MHz
gain-bandwidth, >120dB open-loop gain design. The am-
plifier has an output slew rate of 5V/
s and is capable of
driving capacitive loads with an output RMS current typi-
cally up to 25mA. The amplifier is not capable of sinking
current and therefore must be resistively loaded to do so.
Power Good (PGOOD) Pin (LTC1929-PG Only)
The PGOOD pin is connected to an open drain of a
MOSFET. The MOSFET turns on and pulls the pin low when
the output is not within
±7.5% of its nominal output level
as determined by its resistive feedback divider. When the
output meets the
±7.5% requirement, the MOSFET is
turned off within 10
s and the pin is allowed to be pulled
up by an external source.
Short-Circuit Detection
The RUN/SS capacitor is used initially to limit the inrush
current from the input power source. Once the controllers
have been given time, as determined by the capacitor on
the RUN/SS pin, to charge up the output capacitors and
provide full load current, the RUN/SS capacitor is then
used as a short-circuit timeout circuit. If the output voltage
falls to less than 70% of its nominal output voltage the
RUN/SS capacitor begins discharging assuming that the
output is in a severe overcurrent and/or short-circuit
condition. If the condition lasts for a long enough period
as determined by the size of the RUN/SS capacitor, the
controller will be shut down until the RUN/SS pin voltage
is recycled. This built-in latchoff can be overidden by
providing a current >5
A at a compliance of 5V to the
RUN/SS pin. This current shortens the soft-start period
but also prevents net discharge of the RUN/SS capacitor
during a severe overcurrent and/or short-circuit condi-
tion. Foldback current limiting is activated when the output
voltage falls below 70% of its nominal level whether or not
the short-circuit latchoff circuit is enabled.
APPLICATIO S I FOR ATIO
WU
U
The basic LTC1929 application circuit is shown in Figure 1
on the first page. External component selection is driven
by the load requirement, and begins with the selection of
RSENSE1, 2. Once RSENSE1, 2 are known, L1 and L2 can be
chosen. Next, the power MOSFETs and D1 and D2 are
selected. The operating frequency and the inductor are
chosen based mainly on the amount of ripple current.
Finally, CIN is selected for its ability to handle the input
ripple current (that PolyPhase
TM operation minimizes) and
COUT is chosen with low enough ESR to meet the output
ripple voltage and load step specifications (also minimized
with PolyPhase). Current mode architecture provides in-
herent current sharing between output stages. The circuit
shown in Figure 1 can be configured for operation up to an
input voltage of 28V (limited by the external MOSFETs).
RSENSE Selection For Output Current
RSENSE1, 2 are chosen based on the required output
current. The LTC1929 current comparator has a maxi-
mum threshold of 75mV/RSENSE and an input common
mode range of SGND to 1.1( INTVCC). The current com-
parator threshold sets the peak inductor current, yielding
a maximum average output current IMAX equal to the peak
value less half the peak-to-peak ripple current,
IL.
Allowing a margin for variations in the LTC1929 and
external component values yields:
RSENSE = 2(50mV/IMAX)
PolyPhase is a trademark of Linear Technology Corporation.
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