LTC3736EUF#TR Linear Technology, LTC3736EUF#TR Datasheet - Page 20

• Manufacturer Part Number: LTC3736EUF#TR
• Description: IC CTRLR SW SYNC DUAL 2PH 24QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3736EUF#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 9.8 V
• Current - Output: 1A
• Frequency - Switching: 550kHz ~ 750kHz
• Voltage - Input: 2.75 ~ 9.8 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
If the external clock frequency is greater than the internal
oscillator’s frequency, f
tinuously from the phase detector output, pulling up the
PLLLPF pin. When the external clock frequency is less than
f
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 differ-
ence. The voltage on the PLLLPF pin is adjusted until the
phase and frequency of the internal and external oscilla-
tors are identical. At the stable operating point, the phase
detector output is high impedance and the filter capacitor
C
The loop filter components, C
current pulses from the phase detector and provide a
stable input to the voltage-controlled oscillator. The filter
components C
acquires lock. Typically R
0.01µF.
Typically, the external clock (on SYNC/FCB pin) input high
level is 1.6V, while the input low level is 1.2V.
Table 2 summarizes the different states in which the
PLLLPF pin can be used.
Table 2
PLLLPF PIN
0V
Floating
V
RC Loop Filter
LTC3736
20
OSC
IN
LP
, current is sunk continuously, pulling down the PLLLPF
holds the voltage.
LP
SYNC/FCB PIN
Clock Signal
DC Voltage
DC Voltage
DC Voltage
and R
U
OSCILLATOR
OSC
EXTERNAL
LP
U
LP
, then current is sourced con-
determine how fast the loop
= 10k and C
LP
Phase-Locked to External Clock
and R
SYNC/
FCB
W
FREQUENCY
LP
300kHz
550kHz
750kHz
Figure 9. Phase-Locked Loop Block Diagram
, smooth out the
FREQUENCY
LP
DETECTOR
DIGITAL
PHASE/
is 2200pF to
U
2.4V
Auxiliary Winding Control Using SYNC/FCB Pin
The SYNC/FCB 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 0.6V
threshold, continuous mode operation is forced.
During continuous mode, current flows continuously in
the transformer primary. The auxiliary winding draws
current only when the bottom, synchronous N-channel
MOSFET is on. When primary load currents are low and/or
the V
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 FCB input pin removes the
requirement that power must be drawn from the trans-
former primary 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 V
in Figure 10 by the turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
V
AUX
V
V
PLLLPF
AUX
AUX MIN
R
to the FCB sets a minimum voltage V
IN
LP
OSCILLATOR
AUX
/V
(
≅ (N + 1) V
OUT
3736 F09
will droop. An external resistor divider from
)
C
LP
=
ratio is close to unity, the synchronous
0 6
.
V
OUT
⎛
⎜
⎝
1
+
R
R
6
5
AUX
⎞
⎟
⎠
is normally set as shown
AUX(MIN)
:
3736fa