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參數資料
| 型號： | LTC1773 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Synchronous Step-Down Regulator Controller(同步步降穩(wěn)壓器) |
| 中文描述： | 同步降壓穩(wěn)壓控制器（同步步降穩(wěn)壓器） |
| 文件頁數： | 10/20頁 |
| 文件大?。?/td> | 274K |
| 代理商： | LTC1773 |
LTC1773
10
ratio of any aluminum electrolytic at a somewhat higher
price. Once the ESR requirement for C
OUT
has been met,
the RMS current rating generally far exceeds the
I
RIPPLE(P-P)
requirement.
In surface mount applications multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalum, available in case
heights ranging from 2mm to 4mm. Other capacitor types
include Sanyo OS-CON and POSCAP, Nichicon PL series,
Panisonic SP series and Sprague 593D and 595D series.
Consult the manufacturer for other specific recommenda-
tions.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
V
V
R
R
OUT
=
+
0 8
.
1
2
1
(2)
The external resistive divider is connected to the output as
shown in Figure 3, allowing remote voltage sensing.
APPLICATIO
S I
FOR
ATIO
U
W
U
U
Run/Soft Start Function
The RUN/SS pin is a dual purpose pin that provides the soft
start function and a means to shut down the LTC1773. Soft
start reduces surge currents from V
IN
by gradually
increasing the internal current limit. Power supply se-
quencing can also be accomplished using this pin.
Figure 3. Setting the LTC1773 Output Voltage
An internal 1.5
μ
A current source charges up an external
capacitor C
SS
. When the voltage on RUN/SS reaches 0.7V
the LTC1773 begins operating. As the voltage on RUN/SS
continues to ramp from 0.7V to 1.8V, the I
TH
clamp is also
ramped at a proportionally linear rate. Depending on the
external R
SENSE
used, the peak inductor current, and thus
the internal current limit, rises with the RUN/SS voltage.
The output current thus ramps up slowly, charging the
output capacitor. If RUN/SS has been pulled all the way to
ground, there will be a delay before the current starts
increasing and is given by:
t
C
μ
A
s
F C
)
DELAY
SS
SS
=
=
(
0 7
1 5
0 47
.
.
.
/
μ
Pulling the RUN/SS pin below 0.4V puts the LTC1773 into
a low quiescent current shutdown mode (I
Q
< 10
μ
A). This
pin can be driven directly from logic as shown in Figure 4.
Diode D1 in Figure 4 reduces the start delay but allows C
SS
to ramp up slowly providing the soft start function. This
diode can be deleted if soft start is not needed.
Figure 4. RUN/SS Pin Interfacing
Auxiliary Winding Control Using SYNC/FCB Pin
The SYNC/FCB pin can be used as a secondary feedback to
provide a means of regulating a flyback winding output.
When this pin drops below its ground referenced 0.8V
threshold, continuous mode operation is forced. In con-
tinuous mode, the P-channel main and N-channel syn-
chronous switches are switched continuously regardless
of the load on the main output.
Synchronous switching removes the normal limitation
that power must be drawn from the inductor primary
winding in order to extract power from auxiliary windings.
With continuous synchronous operation, power can be
LTC1773
V
FB
GND
R1
R2
1773 F03
0.8V
≤
V
OUT
≤
8.5V
3.3V OR 5V
RUN/SS
RUN/SS
C
SS
C
SS
D1
1773 F04
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