LTC1627 Linear Technology, LTC1627 Datasheet - Page 9

• Manufacturer Part Number: LTC1627
• Description: Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
The selection of C
resistance (ESR). Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering. The output
ripple V
where f = operating frequency, C
and I
is highest at maximum input voltage since I
with input voltage. For the LTC1627, the general rule for
proper operation is:
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 POSCAP, KEMET T510 and T495 series,
Nichicon PL 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,
allowing remote voltage sensing as shown in Figure 4.
RIPPLE(P-P)
C
V
OUT
OUT
V
OUT
L
= ripple current in the inductor. The output ripple
required ESR < 0.25
OUT
0 8 1
requirement.
is determined by:
I ESR
V
L
OUT
U
is driven by the required effective series
R
R
2
1
4
INFORMATION
U
fC
1
OUT
OUT
W
= output capacitance
OUT
has been met,
L
U
increases
(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 LTC1627.
Soft-start reduces surge currents from V
increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
An internal 2.25 A current source charges up an external
capacitor C
the LTC1627 begins operating. As the voltage on RUN/SS
continues to ramp from 0.7V to 1.8V, the internal current
limit is also ramped at a proportional linear rate. The
current limit begins at 25mA (at V
at the Figure 3 value (V
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 LTC1627 into
a low quiescent current shutdown (I
be driven directly from logic as shown in Figure 5. Diode
D1 in Figure 5 reduces the start delay but allows C
ramp up slowly providing the soft-start function. This
diode can be deleted if soft-start is not needed.
t
DELAY
3.3V OR 5V
Figure 4. Setting the LTC1627 Output Voltage
SS
2 25
. When the voltage on RUN/SS reaches 0.7V
0 7
Figure 5. RUN/SS Pin Interfacing
D1
C
SS
LTC1627
A
RUN/SS
RUN/SS
GND
C
V
SS
FB
0.8V V
1.8V). The output current
RUN/SS
OUT
Q
R1
R2
< 15 A). This pin can
8.5V
1627 F04
LTC1627
IN
0.7V) and ends
RUN/SS
by gradually
C
SS
1627 F05
SS
9
to