Datasheet

LTC3809
18
3809fc
can be lowered and distributed to other frequencies and
their harmonics by modulating the PWM frequency. The
LTC3809’s switching noise (at 550kHz) is spread between
460kHz and 635kHz in spread spectrum modulation opera-
tion. Figure 8 shows the spectral plots of the output (V
OUT
)
noise with/without spread spectrum modulation. Note the
signifi cant reduction in peak output noise (>20dBm).
The spread spectrum modulation operation of the LTC3809
is enabled by setting SYNC/MODE pin to a DC voltage
between 1.35V and several hundred mV below V
IN
by
tying a resistor between SYNC/MODE and V
IN
.
Table 2 summarizes the different states in which the
SYNC/MODE Pin can be used.
Table 2. The States of the SYNC/MODE Pin
SYNC/MODE PIN CONDITION
GND (0V to 0.35V) Forced Continuous Mode
Current Reversal Allowed
V
FB
(0.45V to 1.2V) Pulse-Skipping Mode
No Current Reversal Allowed
Resistor to V
IN
(1.35V to V
IN
– 0.5V)
Spread Spectrum Modulation
Pulse Skipping at Light Loads
No Current Reversal Allowed
V
IN
Burst Mode Operation
No Current Reversal Allowed
Feedback Resistors Regulate an Auxiliary Winding
External Clock Signal Enable Phase-Locked Loop
(Synchronize to External Clock)
Pulse Skipping at Light Load
No Current Reversal Allowed
Fault Condition: Short-Circuit and Current Limit
If the LTC3809’s load current exceeds the short-circuit
current limit (I
SC
), which is set by the short-circuit sense
threshold (ΔV
SC
) and the on resistance (R
DS(ON)
) of
bottom N-channel MOSFET, the top P-channel MOSFET
is turned off and will not be turned on at the next clock
cycle unless the load current decreases below I
SC
. In this
case, the controller’s switching frequency is decreased
and the output is regulated by short-circuit (current limit)
protection.
In a hard short (V
OUT
= 0V), the top P-channel MOSFET
is turned off and kept off until the short-circuit condition
is cleared. In this case, there is no current path from
input supply (V
IN
) to either V
OUT
or GND, which prevents
excessive MOSFET and inductor heating.
Low Input Supply Voltage
Although the LTC3809 can function down to below 2.4V,
the maximum allowable output current is reduced as V
IN
decreases below 3V. Figure 9 shows the amount of change
as the supply is reduced down to 2.4V. Also shown is the
effect on V
REF
.
APPLICATIONS INFORMATION
V
OUT
Spectrum without Spread Spectrum Modulation
START FREQ: 400kHz
RBW: 100Hz
STOP FREQ: 700kHz
NOISE (dBm)
–10dBm/DIV
3809 F08a
V
OUT
Spectrum with Spread Spectrum Modulation
(C
SSM
= 2200pF)
Figure 8. Spectral Response of Spread Spectrum Modulation
START FREQ: 400kHz
RBW: 100Hz
STOP FREQ: 700kHz
NOISE (dBm)
–10dBm/DIV
3809 F08b