Datasheet

LTC3824
8
3824fg
applicaTions inForMaTion
Burst Mode Operation
The LTC3824 can be configured for Burst Mode operation to
enhance light load efficiency (only 40µA quiescent current)
and extend battery run time by leaving the SYNC/MODE
pin open or pulling it higher than 2V. In this mode, when
output load drops the loop control voltage V
C
also drops
and when V
C
reaches approximately 0.9V at low duty cycle
the LTC3824 goes into sleep mode with the switch turned
off. During sleep mode the output voltage drops and V
C
rises up. When V
C
goes up to around 70mV the LTC3824
will turn on the switch and the burst cycle repeats. If the
SYNC/MODE pin is grounded the Burst Mode operation
will be disabled and the LTC3824 skips cycles at light load.
Oscillation Frequency Setting and Synchronization
The switching frequency of the LTC3824 can be set up to
600kHz by a resistor, R
FREQ
, from the R
SET
pin to ground.
For 200kHz, R
FREQ
= 392k. See the Switching Frequency
vs R
FREQ
graph in the Typical Performance Characteris-
tics section. With a 100ns one-shot timer on-chip, the
LTC3824 provides flexibility on the sync pulse width. The
sync pulse threshold voltage level is about 1.2V.
Short-Circuit Protection
In normal operation when the output voltage is in regulation,
V
FB
is regulated to 0.8V. If the output is shorted to ground
and V
FB
drops below 0.5V the switching frequency will be
reduced to 50kHz to allow the inductor current to discharge
and prevent current runaway. Note that synchronization
is enabled only when V
FB
is above 0.5V.
Soft-Start
During soft-start, the voltage on the SS pin (V
SS
) is the
reference voltage that controls the output voltage and the
output ramps up following V
SS
. The effective range of V
SS
is from 0V to 0.8V. The typical time for the output to reach
the programmed level is:
t
SS
=
C
SS
• 0.8V
5μA
where C
SS
is the capacitor connected from the SS pin to
GND.
Overvoltage Protection
To achieve good output regulation in Burst Mode operation,
an overvoltage comparator, OVP, with a threshold adap-
tive to the V
C
voltage is used to monitor the FB voltage.
In Burst Mode operation with low V
C
voltage, the OVP
threshold is approximately 2% above V
REF
and the V
REF
is also shifted lower by 2% to contain the output ripple
and to keep output regulation constant. As output load
increases, OVP threshold increases with V
C
voltage to up
to 8% above V
REF
.
Shutdown Mode Quiescent Current
When the V
C
pin is pulled down below 25mV the LTC3824
goes into micropower shutdown mode and only draws 7µA.
Output Voltage Programming
With a 0.8V feedback reference voltage, V
REF
, the output
voltage, V
OUT
, is programmed by a resistor divider as
shown in the Block Diagram.
V
OUT
= 0.8V (1+R
F1
/R
F2
)
Current Sense Resistor R
S
and Current Limit
The maximum current the LTC3824 can deliver is deter-
mined by:
I
OUT(MAX)
= 100mV/R
S
– I
RIPPLE
/2
where 100mV is the internal 100mV threshold across V
CC
and V
SENSE
, and I
RIPPLE
is the inductor peak-to-peak ripple
current. R
S
should be placed very close to the power switch
with very short traces. Good kelvin sensing is required for
accurate current limit.