LTC3122_15 Datasheet, PDF (7/26 Pages) Linear Technology – 15V, 2.5A Synchronous Step-Up DC/DC Converter with Output Disconnect

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.
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.
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.
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:
• 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
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.
• Buck Mode (V_OUT < V_IN): I_OUT <5mA: ƒ_PWM/SYNC ≤ 5Hz,
I_OUT ≥ 5mA: ƒ_PWM/SYNC ≤ 5kHz
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.
3122fa
For more information www.linear.com/LTC3122
7