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參數(shù)資料
| 型號： | LTC3708 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | No RSENSE Low EMI, Synchronous DC/DC Controller with Output Tracking |
| 中文描述： | 無檢測電阻低EMI，同步DC / DC控制器的輸出跟蹤 |
| 文件頁數(shù)： | 20/28頁 |
| 文件大?。?/td> | 372K |
| 代理商： | LTC3708 |
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LTC3808
3808f
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
V
R
R
AUX MIN
(
)
.
=
+
0 4
1
6
5
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
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
GND (0V to 0.35V)
CONDITION
Forced Continuous Mode
Current Reversal Allowed
Pulse Skipping Mode
No Current Reversal Allowed
Spread Spectrum Modulation
Pulse Skipping at Light Loads
No Current Reversal Allowed
Burst Mode Operation
No Current Reversal Allowed
Regulate an Auxiliary Winding
Enable Phase-Locked Loop
(Synchronize to External Clock)
Pulse Skipping at Light Load
No Current Reversal Allowed
V
FB
(0.45V to 1.2V)
Resistor to V
IN
(1.35V to V
IN
– 0.5V)
V
IN
Feedback Resistors
External Clock Signal
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.
APPLICATIU
W
U
U
Figure 9. Spectral Response of Spread Spectrum Modulation
V
OUT
Spectrum with Spread Spectrum Modulation
(C
SSM
= 2200pF)
V
OUT
Spectrum without Spread Spectrum Modulation
START FREQ: 400kHz
RBW: 100Hz
STOP FREQ: 700kHz
NOISE (dBm)
–10dBm/DIV
3808 F09a
START FREQ: 400kHz
RBW: 100Hz
STOP FREQ: 700kHz
NOISE (dBm)
–10dBm/DIV
3808 F09b
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