8
LT1680
Basic Control Loop
The LT1680 uses a constant frequency, current mode
architecture. The timing of the IC is provided through an
internal oscillator circuit that can be synchronized to an
external clock and is programmable to operate at frequen-
cies up to 200kHz. The oscillator creates a modified
sawtooth wave at its timing node (C
T
) with a slow charge,
rapid discharge characteristic.
During typical boost converter operation, the MOSFET
switch is enabled at the start of each oscillator cycle. The
switch stays enabled until the current through the switched
inductor, sensed via the voltage across a series sense
resistor (R
SENSE
), is sufficient to trip the current com-
parator (IC1) and reset the RS latch. When the switch is
disabled, the inductor current is redirected to the supply
output. If the current comparator threshold is not reached
throughout the entire oscillator charge period, the RS
latch is bypassed and the main switch is disabled during
the oscillator discharge time. This “minimum off time”
protects the switch, and is typically about 1µs.
The current comparator trip threshold is set on the V
C
pin,
which is the output of a transconductance amplifier, or
error amplifier (EA). The error amplifier integrates the
difference between a feedback voltage (on the V
FB
pin) and
an internal bandgap generated reference voltage of 1.25V,
forming a signal that represents required load current. If
the supplied current is insufficient for a given load, the
output will droop, thus reducing the feedback voltage. The
error amplifier responds by forcing current out of the V
C
pin, increasing the current comparator threshold. Thus,
the circuit will servo until the provided current is equal to
the required load and the average output voltage is at the
value programmed by the feedback resistors.
Input Average Current Limit
The output of the sense amplifier is monitored by a single
pole integrator comprised of an external capacitor on the
I
AVG
pin and an output impedance of approximately 50kΩ.
If this averaged value signal exceeds a level corresponding
to 120mV across the external sense resistor, the current
comparator threshold is clamped and cannot continue to
rise in response to the error amplifier. Thus, if average
input current requirements exceed 120mV/R
SENSE
, the
supply will current limit and the output voltage will fall out
of regulation. The average current limit circuit monitors
the sense amplifier output without slope compensation or
ripple current contributions. Therefore, the average input
current limit threshold is unaffected by duty cycle.
Undervoltage Lockout
The LT1680 employs an undervoltage lockout circuit
(UVLO) that monitors the 12V
IN
supply rail. This circuit
disables the output drive capability of the LT1680 if the
12V supply drops below 9V. Unstable mode switching is
prevented through 350mV of UVLO threshold hysteresis.
Shutdown
The LT1680 can be put into low current shutdown by
pulling the RUN/SHDN pin low, disabling all circuit func-
tions. The shutdown threshold is a bandgap referred
voltage of 1.25V typical. Use of a precision threshold on
the shutdown circuit enables use of this pin for under-
voltage protection of the V
IN
supply and/or power supply
sequencing.
Soft Start
The LT1680 incorporates a soft start function that oper-
ates by slowly increasing current limit. This limit is
controlled by internally clamping the V
C
pin to a low
voltage that climbs with time as an external capacitor on
the SS pin is charged with about 8µA. This forces a
graceful climb of output current source capability, and
thus a graceful increase in output voltage until steady-
state regulation is achieved. The soft start timing capaci-
tor is clamped to ground during shutdown and during
undervoltage lockout, yielding a graceful output recovery
from either condition.
5V Internal Reference
Power for the oscillator timing elements and most other
internal LT1680 circuits is derived from an internal 5V
reference, accessible at the 5V
REF
pin. This supply pin
can be loaded with up to 10mA DC (20mA pulsed) for
convenient biasing of local elements such as control
logic, etc.
OPERATIO
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