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參數資料
| 型號： | LTC3809EMS#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 825 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | PLASTIC, MSOP-10 |
| 文件頁數： | 9/24頁 |
| 文件大?。?/td> | 266K |
| 代理商： | LTC3809EMS#PBF |
17
LTC3809
3809f
If the external clock frequency is greater than the internal
oscillator’s frequency, fOSC, then current is sourced con-
tinuously from the phase detector output, pulling up the
PLLLPF pin. When the external clock frequency is less
than fOSC, current is sunk continuously, pulling down the
PLLLPF pin. If the external and internal frequencies are the
same but exhibit a phase difference, the current sources
turn on for an amount of time corresponding to the phase
difference. The voltage on the PLLLPF pin is adjusted until
the phase and frequency of the internal and external
oscillators are identical. At the stable operating point, the
phase detector output is high impedance and the filter
capacitor CLP holds the voltage.
The loop filter components, CLP and RLP, smooth out the
current pulses from the phase detector and provide a
stable input to the voltage-controlled oscillator. The filter
components CLP and RLP determine how fast the loop
acquires lock. Typically RLP = 10k and CLP is 2200pF to
0.01
F.
Typically, the external clock (on SYNC/MODE pin) input
high level is 1.6V, while the input low level is 1.2V.
Table 1 summarizes the different states in which the
PLLLPF pin can be used.
Table 1. The States of the PLLLPF Pin
PLLLPF PIN
SYNC/MODE PIN
FREQUENCY
0V
DC Voltage (<1.2V or VIN)
300kHz
Floating
DC Voltage (<1.2V or VIN)
550kHz
VIN
DC Voltage (<1.2V or VIN)
750kHz
RC Loop Filter Clock Signal
Phase-Locked
to External Clock
Filter Caps
DC Voltage (>1.35V and <VIN – 0.5V)
Spread Spectrum
460kHz to 635kHz
Auxiliary Winding Control Using SYNC/MODE Pin
The SYNC/MODE pin can be used as an auxiliary feedback
to provide a means of regulating a flyback winding output.
When this pin drops below its ground-referenced 0.4V
threshold, continuous mode operation is forced.
During continuous mode, current flows continuously in
the transformer primary side. The auxiliary winding draws
current only when the bottom synchronous N-channel
MOSFET is on. When primary load currents are low and/or
the VIN/VOUT ratio is close to unity, the synchronous
MOSFET may not be on for a sufficient amount of time to
transfer power from the output capacitor to the auxiliary
load. Forced continuous operation will support an auxil-
iary winding as long as there is a sufficient synchronous
MOSFET duty factor. The SYNC/MODE input pin removes
the requirement that power must be drawn from the
transformer primary side in order to extract power from
the auxiliary winding. With the loop in continuous mode,
the auxiliary output may nominally be loaded without
regard to the primary output load.
The auxiliary output voltage VAUX is normally set, as
shown in Figure 7, by the turns ratio N of the transformer:
VAUX = (N + 1) VOUT
However, if the controller goes into pulse skipping opera-
tion and halts switching due to a light primary load current,
then VAUX will droop. An external resistor divider from
VAUXtotheSYNC/MODEsetsaminimumvoltageVAUX(MIN):
VV
R
AUX MIN
()
.
=+
04
1
6
5
If VAUX drops below this value, the SYNC/MODE voltage
forces temporary continuous switching operation until
VAUX is again above its minimum.
APPLICATIO S I FOR ATIO
WU
UU
Figure 7. Auxiliary Output Loop Connection
LTC3809
+
R6
R5
1
F
VOUT
VAUX
COUT
L1
1:N
SYNC/MODE
BG
SW
TG
3809 F07
VIN
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
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