ltc3717-1 Linear Technology Corporation, ltc3717-1 Datasheet - Page 14

• Manufacturer Part Number: ltc3717-1
• Description: Wide Operating Range, No Rsense Step-down Controller For Ddr/qdr Memory Termination
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3717-1
14
After the controller has been started and given adequate
time to charge up the output capacitor, C
short-circuit timer. After the RUN/SS pin charges above
4V, if the output voltage falls below 75% of its regulated
value, then a short-circuit fault is assumed. A 1.8 A cur-
rent then begins discharging C
persists until the RUN/SS pin drops to 3.5V, then the con-
troller 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 operation.
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. 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
fault and also shortens the soft-start period. Using a
resistor to V
increases shutdown current. Connecting a resistor to
INTV
shutdown current, but requires a diode to isolate C
pull-up network must be able to pull RUN/SS above the
4.2V maximum threshold of the latchoff circuit and over-
come the 4 A maximum discharge current.
INTV
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3717-1. The INTV
50mA RMS and must be bypassed to ground with a
minimum of 4.7 F tantalum or other low ESR capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers. Applica-
tions using large MOSFETs with a high input voltage and
C
SS
CC
CC
> C
Regulator
as shown in Figure 6b eliminates the additional
OUT
IN
as shown in Figure 6a is simple, but slightly
V
OUT
U
R
SENSE
SS
U
be made large enough to guar-
(10
– 4
SS
CC
. If the fault condition
W
[F/V s])
pin can supply up to
SS
is used as a
SS
U
during a
SS
SS
. Any
has
high frequency of operation may cause the LTC3717-1 to
exceed its maximum junction temperature rating or RMS
current rating. Most of the supply current drives the
MOSFET gates unless an external EXTV
In continuous mode operation, this current is I
f(Q
estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3717EUH-1
is limited to less than 14mA from a 30V supply:
For larger currents, consider using an external supply with
the EXTV
EXTV
The EXTV
and control power from the output or another external
source during normal operation. Whenever the EXTV
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV
pin to INTV
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV
ing list summarizes the possible connections for EXTV
1. EXTV
internal 5V regulator.
2. EXTV
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
will start-up using the internal linear regulator until the
boosted output supply is available.
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
T
g(TOP)
J
CC
= 70 C + (14mA)(30V)(34 C/W) = 84.3 C
CC
CC
CC
Connection
CC
CC
CC
+ Q
CC
grounded. INTV
connected to an output derived boost network.
connected to an external supply. A high effi-
pin and ensure that EXTV
pin.
pin can be used to provide MOSFET gate drive
. INTV
g(BOT)
CC
). The junction temperature can be
power is supplied from EXTV
CC
is always powered from the
CC
CC
V
source is used.
CC
. The follow-
sn37171 37171fs
GATECHG
CC
until
CC
CC
CC
=
: