LTC3422
6
3422fa
PI FU CTIO S
UUU
SW (Pin 1): Switch Pin for the Inductor Connection.
Minimize trace length between SW and inductor. For
discontinuous inductor current, a controlled impedance is
internally connected from SW to V
IN
to eliminate high
frequency ringing, reducing EMI radiation.
V
IN
(Pin 2): Input Supply Voltage. Connect V
IN
to the input
supply and decouple with a 4.7µF or larger ceramic
capacitor as close to V
IN
as possible.
BURST (Pin 3): Burst Mode
Threshold Adjust. Automatic
Burst Mode Operation: A resistor/capacitor combination
from BURST to ground programs the average load current
at which automatic Burst Mode operation is exited, ac-
cording to the formula:
R
I
B
EXITBURST
=
12
where R
B
is in kΩ and I
EXITBURST
is in amps
C
CV
B
OUT OUT
≥
•
,64 000
where C
B(MIN)
and C
OUT
are in µF.
Please refer to the Burst Mode Output Current Threshold
vs R
BURST
Typical Performance Chacteristic curves.
Note that during Burst Mode operation the peak inductor
current will be approximately 600mA and return to zero on each
cycle. In Burst Mode operation the frequency is variable,
providing a significant efficiency improvement at light loads.
The LTC3422 only allows Burst Mode operation to be entered
once V
OUT
exceeds approximately 2.2V.
Manually Implementing Burst Mode Operation: Ground
BURST to force Burst Mode operation or connect it to V
OUT
to
force fixed frequency PWM mode. Note that BURST must not
be pulled higher than V
OUT
.
SS (Pin 4): Soft-Start. Connect a capacitor from SS 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 2.4µA. The active
range of SS is from 0.8V to 1.6V.
SHDN (Pin 5): Shutdown Input. Less than 250mV on SHDN
shuts down the LTC3422. Placing 1V or more on SHDN enables
the LTC3422. Once V
OUT
exceeds 2.2V, hysteresis is applied
to this pin (500nA exits the pin) allowing it to operate at a logic
high while the battery can drop to 500mV.
FB (Pin 6): Feedback Input to Error Amplifier. Connect the V
OUT
to ground resistor divider tap here. The output voltage can be
adjusted from 2.25V to 5.25V according to the formula:
V
RR
R
OUT
=
+
1 216
12
2
.•
V
C
(Pin 7): Error Amp Output. A frequency compensation
network is connected from V
C
to ground to compensate the
loop. See the section “Closing the Feedback Loop” for guide-
lines.
R
T
(Pin 8): Frequency Adjust Input. Connect a resistor to ground
to program the oscillator frequency according to the formula:
f
R
OSC
T
=
28
where f
OSC
is in MHz and R
T
is in kΩ.
SYNC (Pin 9): Oscillator Synchronization Input. A clock pulse
width of 100ns to 2µs is required to synchronize the internal
oscillator. If not used, SYNC should be grounded.
V
OUT
(Pin 10): Output of the synchronous rectifier and
bootstrapped power source for the LTC3422. A ceramic capaci-
tor of at least 10µF is required and should be located as close to
V
OUT
and the power ground plane as possible.
Exposed Pad (Pin 11): Signal and Power Ground for the
LTC3422. This pin MUST be soldered to the PCB ground plane
for electrical contact and rated thermal performance.
