LTC3122
7
3122fa
For more information www.linear.com/LTC3122
pin FuncTions
SW (Pin 1): Switch Pin. Connect an inductor from this
pin to V
IN
. Keep PCB trace lengths as short and wide as
possible to reduce EMI and voltage overshoot. When V
OUT
≥ V
IN
+ 2V, an internal anti-ringing resistor is connected
between SW and V
IN
after the inductor current has dropped
to near zero, to minimize EMI. The anti-ringing resistor is
also activated in shutdown and during the sleep periods
of Burst Mode operation.
PGND (Pins 2, 13): Power Ground. When laying out your
PCB, provide a short, direct path between PGND and the
output capacitor and tie directly to the ground plane. The
exposed pad is ground and must be soldered to the PCB
ground plane for rated thermal performance.
V
IN
(Pin 3): Input Supply Pin. The device is powered from
V
IN
unless V
OUT
exceeds V
IN
and V
IN
is less than 3V. Place
a low ESR ceramic bypass capacitor of at least 4.7µF from
V
IN
to PGND. X5R and X7R dielectrics are preferred for
their superior voltage and temperature characteristics.
PWM/SYNC (Pin 4): Burst Mode Operation Select and
Oscillator Synchronization. Do not leave this pin floating.
• PWM/SYNC = High. Disable Burst Mode Operation and
maintain low noise, constant frequency operation.
• PWM/SYNC = Low. The converter operates in Burst
Mode operation, independent of load current.
• PWM/SYNC = External CLK. The internal oscillator is
synchronized to the external CLK signal. Burst Mode
operation is disabled. A clock pulse width between
100ns and 2µs is required to synchronize the oscillator.
An external resistor must be connected between RT
and GND to program the oscillator slightly below the
desired synchronization frequency.
In non-synchronized applications, repeated clocking of
the PWM/SYNC pin to affect an operating mode change
is supported with these restrictions:
• Boost Mode (V
OUT
> V
IN
): I
OUT
<500µA: ƒ
PWM/SYNC
≤
100Hz, I
OUT
≥ 500µA: ƒ
PWM/SYNC
≤ 5kHz
• Buck Mode (V
OUT
< V
IN
): I
OUT
<5mA: ƒ
PWM/SYNC
≤ 5Hz,
I
OUT
≥ 5mA: ƒ
PWM/SYNC
≤ 5kHz
V
CC
(Pin 5): V
CC
Regulator Output. Connect a low-ESR
filter capacitor of at least 4.7µF from this pin to GND to
provide a regulated rail approximately equal to the lower of
V
IN
and 4.25V. When V
OUT
is higher than V
IN
, and V
IN
falls
below 3V, V
CC
will regulate to the lower of approximately
V
OUT
and 4.25V. A UVLO event occurs if V
CC
drops below
1.6V. Switching is inhibited, and a soft-start is initiated
when V
CC
returns above 1.7V.
RT (Pin 6): Frequency Adjust Pin. Connect an external
resistor (R
T
) from this pin to SGND to program the oscil-
lator frequency according to the formula:
R
T
= 57.6/ƒ
OSC
where ƒ
OSC
is in MHz and R
T
is in kΩ.
VC (Pin 7): Error Amplifier Output. A frequency compen-
sation network is connected to this pin to compensate
the control loop. See Compensating the Feedback Loop
section for guidelines.
FB (Pin 8): Feedback Input to the Error Amplifier. Connect
the resistor divider tap to this pin. Connect the top of the
divider to V
OUT
and the bottom of the divider to SGND.
The output voltage can be adjusted from 2.2V to 15V ac-
cording to this formula:
V
OUT
= 1.202V • (1 + R1/R2)
SD (Pin 9): Logic Controlled Shutdown Input. Bringing this
pin above 1.6V enables normal, free-running operation,
forcing this pin below 0.25V shuts the LTC3122 down, with
quiescent current below 1μA. Do not leave this pin floating.
SGND (Pin 10): Signal Ground. When laying out a PC
board, provide a short, direct path between SGND and
the (–) side of the output capacitor.
V
OUT
(Pin 11): Output Voltage Sense and the Source of
the Internal Synchronous Rectifier MOSFET. Driver bias
is derived from V
OUT
. Connect the output filter capacitor
from V
OUT
to PGND, as close to the IC as possible. A
minimum value of 10µF ceramic is recommended. V
OUT
is disconnected from V
IN
when SD is low.
CAP (Pin 12): Serves as the Low Reference for the Syn-
chronous Rectifier Gate Drive. Connect a low ESR filter
capacitor (typically 100nF) from this pin to V
OUT
to provide
an elevated ground rail, approximately 5.6V below V
OUT
,
used to drive the synchronous rectifier.
