LTC1909-8 LINER [Linear Technology], LTC1909-8 Datasheet - Page 22

• 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
LTC1909-8
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 LTC1909-8 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 LTC1909-8 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.
External Gate Drive Buffers
The LTC1909-8 drivers are adequate for driving up to
about 30nC into MOSFET switches with RMS currents of
50mA. Applications with larger MOSFET switches or oper-
ating at frequencies requiring greater RMS currents will
benefit from using external gate drive buffers such as the
22
T
g(TOP)
J
CC
= 70 C + (14mA)(30V)(130 C/W) = 125 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
U
). The junction temperature can be
power is supplied from EXTV
U
CC
is always powered from the
W
CC
CC
V
source is used.
IN
. The follow-
U
GATECHG
CC
until
CC
CC
CC
=
:
LTC1693. Alternately, the external buffer circuit shown in
Figure 5 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
LTC1909-8 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC1909-8 into a low quiescent current shutdown
(I
current source to charge up the external timing capacitor
C
is a delay before starting of about:
When the voltage on RUN/SS reaches 1.5V, the LTC1909-8
begins operating with a clamp on I
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
is raised until its full 2.4V range is available. This takes an
additional 1.3s/ F, during which the load current is folded
back until the output reaches 75% of its final value. The pin
can be driven from logic (Figures 6a or 6b) or from the
CPUON pin (Figures 6c and 6d). Diode D1 reduces the start
delay while allowing C
start function.
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
current then begins discharging C
persists until the RUN/SS pin drops to 3.5V, then the
TG
SS
Q
t
< 30 A). Releasing the pin allows an internal 1.2 A
. If RUN/SS has been pulled all the way to ground, there
DELAY
10
BOOST
SW
Figure 5. Optional External Gate Driver
Q1
FMMT619
Q2
FMMT720
1 2
1 5
.
.
CC
V
A
voltage of about 6V.
C
GATE
OF M1
SS
SS
to charge up slowly for the soft-
1 3
. /
BG
s F C
SS
. If the fault condition
TH
10
SS
INTV
PGND
of approximately
Q3
FMMT619
Q4
FMMT720
CC
SS
is used as a
19098 F05
GATE
OF M2
19098f
TH