LTC3122
10
3122fa
For more information www.linear.com/LTC3122
operaTion
Internal Current Limit
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. Peak switch
current is limited to 3.5A, independent of input or output
voltage, except when V
OUT
is below 1.5V, resulting in the
current limit being approximately half of the nominal peak.
Lossless current sensing converts the peak current sig
-
nal of the N-channel MOSFET switch into a voltage that
is summed with the internal slope compensation. The
summed signal is compared to the error amplifier output
to provide a peak current control command for the PWM.
Zero Current Comparator
The zero current comparator monitors the inductor current
being delivered to the output and shuts off the synchro
-
nous rectifier when this current reduces to approximately
50mA. This prevents the inductor current from reversing
in polarity, improving efficiency at light loads.
Oscillator
T
he internal oscillator is programmed to the desired switch
-
ing frequency with an external resistor from the RT pin to
GND according to the following formula:
ƒ
OSC
(MHz) =
57.6
R
(kΩ)
⎛
⎜
⎞
⎟
The oscillator also can be synchronized to an external
frequency by applying a pulse train to the PWM/SYNC pin.
An external resistor must be connected between RT and
GND to program the oscillator to a frequency approximately
25% below that of the externally applied pulse train used
for synchronization. R
T
is selected in this case according
to this formula:
R
T
(kΩ) =
73.2
ƒ
(MHz)
⎛
⎜
⎞
⎟
Output Disconnect
The LTC3122’s output disconnect feature eliminates body
diode conduction of the internal P-channel MOSFET
rectifier. This allows for V
OUT
to discharge to 0V during
shutdown, and draw no current from the input source. It
also allows for inrush current limiting at turn-on, minimiz
-
ing surge currents seen by the input supply. Note that to
obtain the
advantages of output disconnect, there must
not be an external Schottky diode connected between SW
and V
OUT
. The output disconnect feature also allows V
OUT
to be pulled high, without reverse current being backfed
into the power source connected to V
IN
.
Shutdown
The boost converter is disabled by pulling SD below 0.25V
and enabled by pulling SD above 1.6V. Note that SD can
be driven above V
IN
or V
OUT
, as long as it is limited to less
than the absolute maximum rating.
Thermal Shutdown
If the die temperature exceeds 170°C typical, the LTC3122
will go into thermal shutdown (TSD). All switches will be
turned off until the die temperature drops by approximately
7°C, when the device re-initiates a soft-start and switching
can resume.
Boost Anti-Ringing Control
When V
OUT
≥ V
IN
+ 2V, the anti-ringing control connects
a resistor across the inductor to damp high frequency
ringing on the SW pin during discontinuous current mode
operation when the inductor current has dropped to near
zero. Although the ringing of the resonant circuit formed
by L and C
SW
(capacitance on SW pin) is low energy, it
can cause EMI radiation.
V
CC
Regulator
An internal low dropout regulator generates the 4.25V
(nominal) V
CC
rail from V
IN
or V
OUT
, depending upon
operating conditions. V
CC
is supplied from V
IN
when V
IN
is greater than 3.5V, otherwise the greater of V
IN
and V
OUT
is used. The V
CC
rail powers the internal control circuitry
and power MOSFET gate drivers of the LTC3122. The V
CC
regulator is disabled in shutdown to reduce quiescent
current and is enabled by forcing the SD pin above its
threshold. A 4.7µF or larger capacitor must be connected
between V
CC
and SGND.
