LTC1773EMS Linear Technology, LTC1773EMS Datasheet - Page 10

• Manufacturer Part Number: LTC1773EMS
• Description: IC CTRLR DC/DC SYNC STPDN 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1773EMS

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 8.5 V
• Current - Output: 6A
• Frequency - Switching: 550kHz
• Voltage - Input: 2.65 ~ 8.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
LTC1773
10
The I
optimized to provide stable, high performance transient
response regardless of the output capacitors selected.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR/size
ratio of any aluminum electrolytic at a somewhat higher
price. Once the ESR requirement for C
the RMS current rating generally far exceeds the
I
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:
The external resistive divider is connected to the output as
shown in Figure 3, allowing remote voltage sensing.
RIPPLE(P-P)
V
OUT
TH
pin OPTI-LOOP compensation components can be
Figure 3. Setting the LTC1773 Output Voltage
=
0 8 1
requirement.
.
V
⎛
⎜
⎝
LTC1773
+
U
R
R
2
1
GND
⎞
⎟
⎠
V
FB
INFORMATION
U
0.8V ≤ V
W
OUT
OUT
1773 F03
R1
R2
≤ 8.5V
has been met,
U
(2)
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
increasing the internal current limit. Power supply se-
quencing can also be accomplished using this pin.
An internal 1.5µA current source charges up an external
capacitor C
the LTC1773 begins operating. As the voltage on RUN/SS
continues to ramp from 0.7V to 1.8V, the I
ramped at a proportionally linear rate. Depending on the
external R
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:
Pulling the RUN/SS pin below 0.4V puts the LTC1773 into
a low quiescent current shutdown mode (I
pin can be driven directly from logic as shown in Figure 4.
Diode D1 in Figure 4 reduces the start delay but allows C
to ramp up slowly providing the soft-start function. This
diode can be deleted if soft-start is not needed.
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.
t
DELAY
3.3V OR 5V
SENSE
SS
=
. When the voltage on RUN/SS reaches 0.7V
0 7
Figure 4. RUN/SS Pin Interfacing
1 5
.
D1
.
used, the peak inductor current, and thus
C
µ
RUN/SS
SS
A
=
C
SS
(
0 47
.
s
/
µ
F C
)
SS
TH
IN
Q
< 10µA). This
clamp is also
by gradually
RUN/SS
1773 F04
C
SS
1773fb
SS