LTC3421_15 Datasheet, PDF (6/16 Pages) Linear Technology – 3A, 3MHz Micropower Synchronous Boost Converter with Output Disconnect

LTC3421
PI FU CTIO S
R_T (Pin 5): Connect a resistor to ground to program the
oscillator frequency according to the formula:
f_OSC
=
28,100
R_T
where f_OSC is in kHz and R_T is in kΩ.
SS (Pin 6): Soft-Start Pin. Connect a capacitor from this
pin to ground to set the soft-start time according to the
formula:
t(ms) = C_SS(µF) • 320
The nominal soft-start charging current is 2.5µA. The
active range of SS is from 0.8V to 1.6V.
SYNC (Pin 7): Oscillator Synchronization Pin. A clock
pulse width of 100ns to 2µs is required to synchronize the
internal oscillator. If not used SYNC should be grounded.
I_LIM (Pin 8): Current Limit Adjust Pin. Connect a resistor
from this pin to ground to set the peak current limit thresh-
old for the N-channel MOSFET according to the formula
(note that this is the peak current in the inductor):
I_LIM
=
150
R
where I is in amps and R is in kΩ.
BURST (Pin 9): Burst Mode Threshold Adjust Pin. A
resistor/capacitor combination from this pin to ground
programs the average load current at which automatic
Burst Mode operation is entered, according to the formula:
R_BURST
=
2
I_BURST
where R_BURST is in kΩ and I_BURST is in amps.
C_BURST
≥
C_OUT • V_OUT
10,000
where C_BURST(MIN) and C_OUT are in µF.
For manual control of Burst Mode operation, ground the
BURST pin to force Burst Mode operation or connect it to
V_OUT to force fixed frequency PWM mode. Note that the
BURST pin must not be pulled higher than V_OUT.
6
GND (Pin 10): Signal Ground Pin. Connect to ground plane
near the R_T resistor, error amp compensation compo-
nents and feedback divider.
PGND (Pins 11 to 13): Source Terminal of Power Internal
N-Channel MOSFET.
SW (Pins 14 to 16): Switch Pin for Inductor Connection.
For applications where V_OUT > 4.3V, a Schottky diode from
SW to V_OUT or to a snubber circuit is required to maintain
absolute maximum rating for SW. (see Application Cir-
cuits for 5V).
V_OUT (Pins 17, 19 and 20): The output of the synchronous
rectifier and bootstrapped power source for the IC. A
ceramic bypass capacitor is required to be very close to
the V_OUT and PGND pins of the IC.
V_OUTS (Pin 18): V_OUT Sense Pin. Connect V_OUTS directly to
an output filter capacitor. The top of the feedback divider
network should also be tied to this point.
V_IN (Pin 21): Input Supply Pin. Connect this pin to the
input supply and decouple with at least a 4.7µF ceramic
capacitor.
LBO (Pin 22): Open-Drain Output. This pin pulls low when
the LBI input is below 0.6V. The open-drain output can
sink up to 20mA. During Burst Mode operation LBO is only
active during the time the IC wakes up to service the
output.
LBI (Pin 23): Low-Battery Comparator Input. Typical
threshold voltage is 0.6V with 30mV hysteresis. This
function is enabled when the ENB pin is high. The low-
battery comparator will operate off V_IN or V_OUT, whichever
is greater.
V_C (Pin 24): Error Amp Output. A frequency compensation
network is connected from this pin to ground to compen-
sate the loop. See the section Compensating the Feedback
Loop for guidelines.
Exposed Pad (Pin 25): Ground. This pin must be soldered
to the PCB and is typically connected through the power
GND plane.
3421f