lt1738 Linear Technology Corporation, lt1738 Datasheet - Page 7

• Manufacturer Part Number: lt1738
• Description: Slew Rate Controlled Ultralow Noise Dc/dc Controller
• Manufacturer: Linear Technology Corporation

PI FU CTIO S
Approximately 50 A of current can be sourced from this
pin if V
This pin also controls undervoltage lockout of the gate
drive. If the pin is tied to a zener or voltage source, the gate
drive will not be enabled until V
is tied to V
There is an internal 19V zener tied from this pin to ground
to provide a fail-safe for maximum gate voltage.
Slew Control
CAP (Pin 2): This pin is the feedback node for the external
voltage slewing capacitor. Normally a small 1pf to 5pf
capacitor is connected from this pin to the drain of the
MOSFET.
The voltage slew rate is inversely proportional to this
capacitance and proportional to the current that the part
will sink and source on this pin. That current is inversely
proportional to R
R
rate for the external drive MOSFET during switching. The
minimum resistor value is 3.3k and the maximum value is
68k. The time to slew between on and off states of the
MOSFET current will determine how the di/dt related
harmonics are reduced. This time is proportional to R
and R
Longer times produce a greater reduction of higher fre-
quency harmonics.
R
rate for the drain of the external drive MOSFET. The
minimum resistor value is 3.3k and the maximum value is
68k. The time to slew between on and off states on the
MOSFET drain voltage will determine how dv/dt related
harmonics are reduced. This time is proportional to R
C
rolloff of harmonics. C
CAP to the drain of the MOSFET.
Switch Mode Control
SS (Pin 3): The SS pin allows for ramping of the switch
current threshold at startup. Normally a capacitor is placed
on this pin to ground. An internal 9 A current source will
charge this capacitor up. The voltage on the V
V
CSL
VSL
U
and the input voltage. Longer times produce more
(Pin 15): A resistor to ground sets the current slew
(Pin 16): A resistor to ground sets the voltage slew
S
GCL
(the current sense resistor) and maximum current.
IN
U
< V
, then undervoltage lockout is disabled.
IN
– 0.8V.
VSL
.
U
V
is the equivalent capacitance from
IN
> V
GCL
+ 0.8V. If this pin
C
pin cannot
VSL
CSL
,
exceed the voltage on SS. Thus peak current will ramp up
as the SS pin ramps up. During a short circuit fault the SS
pin will be discharged to ground thus reinitializing soft-
start.
When SS is below the V
closely track the SS pin.
This pin can be left open if not used.
CS (Pin 4): This is the input to the current sense amplifier.
It is used for both current mode control and current
slewing of the external MOSFET. Current sense is accom-
plished via a sense resistor (R
source of the external MOSFET to ground. CS is connected
to the top of R
pin.
The switch maximum operating current will be equal to
0.1V/R
turned off (no slew control).
If CS = 0.22V in addition to the drivers being turned off, V
and SS will be discharged to ground (short-circuit protec-
tion). This will hasten turn off on subsequent cycles.
FB (Pin 9): The feedback pin is used for positive voltage
sensing. It is the inverting input to the error amplifier. The
noninverting input of this amplifier connects internally to
a 1.25V reference.
If the voltage on this pin exceeds the reference by 220mV,
then the output driver will immediately turn off the external
MOSFET (no slew control). This provides for output over-
voltage protection
When this input is below 0.9V then the current sense
blanking will be disabled. This will assist start up.
NFB (Pin 10): The negative feedback pin is used for
sensing a negative output voltage. The pin is connected to
the inverting input of the negative feedback amplifier
through a 100k source resistor. The negative feedback
amplifier provides a gain of –0.5 to the FB pin. The nominal
regulation point would be –2.5V on NFB. This pin should
be left open if not used.
If NFB is being used then overvoltage protection will occur
at 0.44V below the NFB regulation point.
At NFB < –1.8 current sense blanking will be disabled.
S
. At CS = 0.1V, the gate driver will be immediately
S
. Current sense is referenced to the GND
C
clamp voltage the V
S
) connected from the
LT1738
C
pin will
1738fa
7
C