LTC3808 Datasheet, PDF (20/28 Pages) Linear Technology – No RSENSE TM, Low EMI, Synchronous DC/DC Controller with Output Tracking

LTC3808
APPLICATIO S I FOR ATIO
However, if the controller goes into pulse skipping opera-
tion and halts switching due to a light primary load current,
then V_AUX will droop. An external resistor divider from
V_AUX to the SYNC/MODE sets a minimum voltage V_AUX(MIN):
V_AUX(MIN)
=
0.4V
•
⎛
⎝⎜
1+
R6⎞
R5⎠⎟
If V_AUX drops below this value, the SYNC/MODE voltage
forces temporary continuous switching operation until
V_AUX is again above its minimum.
Spread Spectrum Modulation with SYNC/MODE and
PLLLPF Pins
Switching regulators, which operate at fixed frequency,
conduct electromagnetic interference (EMI) to their down-
stream load(s) with high spectral power density at this
fundamental and harmonic frequencies. The peak energy
can be lowered and distributed to other frequencies and
their harmonics by modulating the PWM frequency. The
LTC3808’s switching noise (at 550kHz) is spread between
V_OUT Spectrum without Spread Spectrum Modulation
NOISE (dBm)
–10dBm/DIV
460kHz and 635kHz in spread spectrum modulation op-
eration. Figure 9 shows the spectral plots of the output
(V_OUT) noise with/without spread spectrum modulation.
Note the significant reduction in peak output noise
(>20dBm).
The spread spectrum modulation operation of the LTC3808
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 3 summarizes the different states in which the
SYNC/MODE Pin can be used.
Table 3. 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
START FREQ: 400kHz
RBW: 100Hz
STOP FREQ: 700kHz
3808 F09a
V_OUT Spectrum with Spread Spectrum Modulation
(C_SSM = 2200pF)
NOISE (dBm)
–10dBm/DIV
START FREQ: 400kHz
RBW: 100Hz
STOP FREQ: 700kHz
3808 F09b
Figure 9. Spectral Response of Spread Spectrum Modulation
20
Fault Condition: Short-Circuit and Current Limit
If the LTC3808’s load current exceeds the short-circuit cur-
rent limit (I_SC), which is set by the short-circuit sense thresh-
old (∆V_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.
3808f