LTC1909-8 LINER [Linear Technology], LTC1909-8 Datasheet - Page 23

• Manufacturer Part Number: LTC1909-8
• Description: Wide Operating Range,No RSENSE TM Step-Down DC/DC Controller with SMBus Programming
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
controller turns off both power MOSFETs, shutting down
the converter permanently. The RUN/SS pin must be
actively pulled down to ground in order to restart opera-
tion. If the RUN/SS pin is tied to the CPUON pin, this is
achieved by pulling the VRON pin low or by sending two
Off protocols to the SMBus VID programmer to force the
CPUON pin low.
The overcurrent protection timer requires that the soft-
start timing capacitor C
antee that the output is in regulation by the time C
reached the 4V threshold. In general, this will depend upon
the size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor can
be estimated from:
Generally 0.1 F is more than sufficient.
Overcurrent latchoff operation is not always needed or
desired. Load current is already limited during a short-
circuit by the current foldback circuitry and latchoff opera-
tion can prove annoying during troubleshooting. The
feature can be overridden by adding a pull-up current
greater than 5 A to the RUN/SS pin. The additional current
prevents the discharge of C
shortens the soft-start period. Using a resistor to V
C
3.3V OR 5V
SS
Figure 6. RUN/SS Pin Interfacing with Latchoff Defeated
> C
CPUON
OUT
*OPTIONAL TO OVERRIDE OVERCURRENT LATCHOFF
*OPTIONAL TO OVERRIDE OVERCURRENT LATCHOFF
D1
(6a)
(6c)
V
OUT
V
V
IN
IN
R
R
U
SS
SS
R
*
*
RUN/SS
RUN/SS
SENSE
SS
U
be made large enough to guar-
C
C
SS
SS
(10
SS
CPUON
– 4
during a fault and also
[F/V s])
W
INTV
INTV
CC
CC
R
R
D2*
D2*
SS
SS
(6b)
(6d)
*
*
RUN/SS
RUN/SS
U
19098 F06
SS
C
C
SS
SS
IN
has
as
shown in Figure 6a or 6c is simple, but slightly increases
shutdown current. Connecting a resistor to INTV
shown in Figure 6b and 6d eliminates the additional
shutdown current, but requires a diode to isolate C
pull-up network must be able to maintain RUN/SS above
the 4.2V maximum latchoff threshold and overcome the
4 A maximum discharge current.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC1909-8 circuits:
1. DC I
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode the average output current flows through L, but is
chopped between the top and bottom MOSFETs. If the two
MOSFETs have approximately the same R
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
loss. For example, if R
loss will range from 15mW to 1.5W as the output current
varies from 1A to 10A.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the input
voltage, load current, driver strength and MOSFET
capacitance, among other factors. The loss is significant
at input voltages above 20V and can be estimated from:
3. INTV
and control currents. This loss can be reduced by supply-
ing INTV
efficiency source, such as an output derived boost net-
work or alternate supply if available.
4. C
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
Transition Loss (1.7A
IN
loss. The input capacitor has the difficult job of
2
CC
R losses. These arise from the resistances of the
CC
current. This is the sum of the MOSFET driver
current through the EXTV
DS(ON)
–1
= 0.01 and R
) V
IN
2
I
OUT
LTC1909-8
CC
C
pin from a high
RSS
DS(ON)
L
= 0.005 , the
f
2
R loss and
, then the
23
SS
CC
. Any
19098f
2
as
R