LTC3717EGN Linear Technology, LTC3717EGN Datasheet - Page 12

• Manufacturer Part Number: LTC3717EGN
• Description: IC STEP-DWN CONTRLR SYNC 16-SSOP
• Manufacturer: Linear Technology

Specifications of LTC3717EGN

• Applications: Controller, DDR, QDR
• Voltage - Input: 4 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 2 ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

LTC3717
APPLICATIO S I FOR ATIO
time that the LTC3717 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 300ns. The
minimum off-time limit imposes a maximum duty cycle of
t
due to a dropping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
INTV
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3717. 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
high frequency of operation may cause the LTC3717 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 LTC3717CGN
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
12
ON
T
V
/(t
g(TOP)
J
IN MIN
CC
CC
ON
= 70 C + (14mA)(30V)(130 C/W) = 125 C
(
Regulator
+ t
Connection
CC
CC
CC
+ Q
)
OFF(MIN)
CC
pin and ensure that EXTV
pin.
pin can be used to provide MOSFET gate drive
. INTV
V
g(BOT)
OUT
). If the maximum duty cycle is reached,
CC
U
t
). The junction temperature can be
ON
power is supplied from EXTV
t
t
U
ON
OFF MIN
(
CC
)
pin can supply up to
W
CC
CC
V
source is used.
CC
. The follow-
U
GATECHG
CC
until
CC
CC
CC
=
:
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.
External Gate Drive Buffers
The LTC3717 drivers are adequate for driving up to about
60nC into MOSFET switches with RMS currents of 50mA.
Applications with larger MOSFET switches or operating at
frequencies requiring greater RMS currents will benefit
from using external gate drive buffers such as the LTC1693.
Alternately, the external buffer circuit shown in Figure 4
can be used. Note that the bipolar devices reduce the
signal swing at the MOSFET gate, and benefit from an
increased EXTV
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3717 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC3717 into a low quiescent current shutdown (I
30 A). Releasing the pin allows an internal 1.2 A current
source to charge up the external timing capacitor C
RUN/SS has been pulled all the way to ground, there is a
delay before starting of about:
t
DELAY
TG
CC
CC
CC
10
grounded. INTV
connected to an output derived boost network.
connected to an external supply. A high effi-
Figure 4. Optional External Gate Driver
1 2
BOOST
1 5
.
SW
.
CC
Q1
FMMT619
Q2
FMMT720
V
A
voltage of about 6V.
C
SS
GATE
OF M1
CC
1 3
. /
is always powered from the
BG
s F C
10
SS
INTV
PGND
Q3
FMMT619
Q4
FMMT720
CC
GATE
OF M2
sn3717 3717fs
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SS
Q
. If
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