OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT1683EG#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LT1683
10
1683fd
OPERATION
In noise sensitive applications switching regulators tend
to be ruled out as a power supply option due to their pro-
pensity for generating unwanted noise. When switching
supplies are required due to efficiency or input/output
constraints, great pains must be taken to work around
the noise generated by a typical supply. These steps may
include pre and post regulator filtering, precise synchroni-
zation of the power supply oscillator to an external clock,
synchronizing the rest of the circuit to the power supply
oscillator or halting power supply switching during noise
sensitive operations. The LT1683 greatly simplifies the task
of eliminating supply noise by enabling the design of an
inherently low noise switching regulator power supply.
The LT1683 is a fixed frequency, current mode switching
regulator with unique circuitry to control the voltage and
current slew rates of the output switches. Current mode
control provides excellent AC and DC line regulation and
simplifies loop compensation.
Slew control capability provides much greater control over
the power supply components that can create conducted
and radiated electromagnetic interference. Compliance
with EMI standards will be an easier task and will require
fewer external filtering components.
The LT1683 uses two external N-channel MOSFETs as the
power switches. This allows the user to tailor the drive
conditions to a wide range of voltages and currents.
CURRENT MODE CONTROL
Referring to the Block Diagram. A switching cycle begins
with an oscillator discharge pulse, which resets the RS
flip-flop, turning on one of the external MOSFET drivers.
The switch current is sensed across the external sense
resistor and the resulting voltage is amplified and com-
pared to the output of the error amplifier (V
C
pin). The
driver is turned off once the output of the current sense
amplifier exceeds the voltage on the V
C
pin. In this way
pulse by pulse current limit is achieved. The toggle flip-flop
ensures that the two MOSFETs are enabled on alternate
clock cycles. Internal slope compensation is provided to
ensure stability under high duty cycle conditions.
Output regulation is obtained using the error amp to
set the switch current trip point. The error amp is a
transconductance amplifier that integrates the difference
between the feedback output voltage and an internal 1.25V
reference. The output of the error amp adjusts the switch
current trip point to provide the required load current
at the desired regulated output voltage. This method of
controlling current rather than voltage provides faster
input transient response, cycle-by-cycle current limiting
for better output switch protection and greater ease in
compensating the feedback loop. The V
C
pin is used for
loop compensation and current limit adjustment. During
normal operation the V
C
voltage will be between 0.25V
and 1.27V. An external clamp on V
C
or SS may be used
for lowering the current limit.
The negative voltage feedback amplifier allows for direct
regulation of negative output voltages. The voltage on the
NFB pin gets amplified by a gain of – 0.5 and driven on to
the FB input, i.e., the NFB pin regulates to –2.5V while the
amplifier output internally drives the FB pin to 1.25V as in
normal operation. The negative feedback amplifier input
impedance is 100k (typ) referred to ground.
Soft-Start
Control of the switch current during start-up can be
obtained by using the SS pin. An external capacitor from
SS to ground is charged by an internal 9µA current source.
The voltage on V
C
cannot exceed the voltage on SS. Thus
as the SS pin ramps up the V
C
voltage will be allowed to
ramp up. This will then provide for a smooth increase in
switch maximum current. SS will be discharged as a result
of the CS voltage exceeding the short-circuit threshold of
approximately 0.22V.
Slew Control
Control of output voltage and current slew rates is achieved
via two feedback loops. One loop controls the MOSFET drain
dV/dt and the other loop controls the MOSFET dI/dt.
The voltage slew rate uses an external capacitor between
CAP A or CAP B and the respective MOSFET drain. These
integrating caps close the voltage feedback loop. The
external resistor, R
VSL
, sets the current for the integrator.
LT1683
11
1683fd
The voltage slew rate is thus inversely proportional to both
the value of capacitor and R
VSL
.
The current slew feedback loop consists of the voltage
across the external sense resistor, which is internally ampli-
fied and differentiated. The derivative is limited to a value set
by R
CSL
. The current slew rate is thus inversely proportional
to both the value of sense resistor and R
CSL.
The two control loops are combined internally so that a
smooth transition from current slew control to voltage
slew control is obtained. When turning on, the driver cur-
rent will slew before voltage. When turning off, voltage
will slew before current. In general it is desirable to have
R
VSL
and R
CSL
of similar value.
Internal Regulator
Most of the control circuitry operates from an internal
2.4V low dropout regulator that is powered from V
IN
. The
internal low dropout design allows V
IN
to vary from 2.7V
to 20V with stable operation of the controller. When SHDN
< 1.3V the internal regulator is completely disabled.
5V Regulator
A 5V regulator is provided for powering external circuitry.
This regulator draws current from V
IN
and requires V
IN
to be greater than 6.5V to be in regulation. It can sink or
source 10mA. The output is current limited to prevent
against destruction from accidental short circuits.
Safety and Protection Features
There are several safety and protection features on the
chip. The first is overcurrent limit. Normally the gate
drivers will go low when the output of the internal sense
amplifier exceeds the voltage on the V
C
pin. The V
C
pin is
clamped such that maximum output current is attained
when the CS pin voltage is 0.1V. At that level the outputs
will be immediately turned off (no slew). The effect of
this control is that the output voltage will foldback with
overcurrent.
In addition, if the CS voltage exceeds 0.22V, the V
C
and
SS pins will be discharged to ground also, resetting the
soft-start function. Thus if a short is present this will allow
for faster MOSFET turnoff and less MOSFET stress.
If the voltage on the FB pin exceeds regulation by ap-
proximately 0.22V, the outputs will immediately go low.
The implication is that there is an overvoltage fault.
The voltage on GCL determines two features. The first
is the maximum gate drive voltage. This will protect the
MOSFET gate from overvoltage.
With GCL tied to a Zener or an external voltage source
then the maximum gate driver voltage is approximately
V
GCL
– 0.2V. If GCL is tied to V
IN
, then the maximum
gate voltage is determined by V
IN
and is approximately
V
IN
– 1.6V. There is an internal 19V Zener on the GCL
pin that prevents the gate driver pin from exceeding ap-
proximately 19V.
In addition, the GCL voltage determines undervoltage
lockout of the gate drives. This feature disables the gate
drivers if V
IN
is too low to provide adequate voltage to
turn on the MOSFETs. This is helpful during start-up to
ensure the MOSFETs have sufficient gate drive to saturate.
If GCL is tied to a voltage source or Zener less than 6.8V,
the gate drivers will not turn on until V
IN
exceeds GCL
voltage by 0.8V. For V
GCL
above 6.5V, the gate drives are
ensured to be off for V
IN
< 7.3V and they will be turned
on by V
GCL
+ 0.8V.
If GCL is tied to V
IN
, the gate drivers are always enabled
(undervoltage lockout is disabled).
When driving a push-pull transformer, it is important to
make sure that both drivers are not on at the same time.
Even though runaway cannot occur under such cross
conduction with this chip because current slew is regu-
lated, increased current would be possible. This chip has
opposite gate lockout whereby when one MOSFET is on
the other MOSFET cannot be turned on until the gate of
the first drops below 1V. This ensures that cross conduc-
tion will not occur.
The gate drives have current limits for the drive currents.
If the sink or source current is greater than 300mA then
the current will be limited.
The V5 regulator also has internal current limiting that will
only guarantee ±10mA output current.
OPERATION
LT1683
12
1683fd
OPERATION
There is also an on-chip thermal shutdown circuit that will
turn off the outputs in the event the chip temperature rises
to dangerous levels. Thermal shutdown has hysteresis that
will cause a low frequency (<1kHz) oscillation to occur as
the chip heats up and cools down.
The chip has an undervoltage lockout feature that will
force the gate drivers low in the event that V
IN
drops below
2.5V. This ensures predictable behavior during start-up
and shutdown. SHDN can be used in conjuction with an
external resistor divider to completely disable the part if
the input voltage is too low. This can be used to ensure
adequate voltage to reliably run the converter. See the
section in Applications Information.
Table 1 summarizes these features.
Table 1. Safety and Protection Features
FEATURE FUNCTION EFFECT ON GATE DRIVERS SLEW CONTROL EFFECT ON VC, SS
Maximum Current Fault Turn Off FETs at Maximum
Switch Current (V
SENSE
= 0.1)
Immediately Goes Low Overridden None
Short-Circuit Fault Turn Off FETs and Reset V
C
for Short-Circuit (V
SENSE
= 0.2)
Immediately Goes Low Overridden Discharge V
C
, SS
to GND
Overvoltage Fault Turn Off Drivers If FB > V
REG
+ 0.22V
(Output Overvoltage)
Immediately Goes Low Overridden None
GCL Clamp Set Max Gate Voltage to Prevent
FET Gate Breakdown
Limits Max Voltage None None
Gate Drive
Undervoltage Lockout
Disable Gate Drives When V
IN
Is Too Low. Set Via GCL Pin
Immediately Goes Low Overridden None
Thermal Shutdown Turn Off Drivers If Chip
Temperature Is Too Hot
Immediately Goes Low Overridden None
Opposite Gate Lockout Prevents Opposite Driver from
Turning on Until Driver Is Off
(Cross Conduction in Transformer)
Inhibits Turn On of
Opposite Driver
None None
V
IN
Undervoltage Lockout
Disable Part When V
IN
≅ 2.55V
Immediately Goes Low Overridden None
Gate Drive Source and Sink Current Limit Limit Gate Drive Current Limit Drive Current None None
V5 Source/Sink Current Limit Limit Current from V5 None None None
Shutdown Disable Part When SHDN <1.3V
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26

LT1683EG#TRPBF

Mfr. #:
Buy LT1683EG#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators SR Controlled Ultralow N PP DC/DC Cntr
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet