LTC3736EGN-2#PBF Linear Technology, LTC3736EGN-2#PBF Datasheet - Page 20

• Manufacturer Part Number: LTC3736EGN-2#PBF
• Description: IC CTRLR SW SYNC DUAL 2PH 24SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3736EGN-2#PBF

• 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-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
LTC3736-2
than f
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 fi lter capacitor C
The loop fi lter components, C
current pulses from the phase detector and provide a
stable input to the voltage-controlled oscillator. The fi lter
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
20
IN
OSC
, current is sunk continuously, pulling down
LP
SYNC/FCB PIN
Clock Signal
DC Voltage
DC Voltage
DC Voltage
and R
LP
holds the voltage.
LP
LP
OSCILLATOR
determine how fast the loop
EXTERNAL
= 10k and C
LP
Phase-Locked to External Clock
and R
FREQUENCY
SYNC/
LP
FCB
Figures 9. Phase-Locked Loop Block Diagram
300kHz
550kHz
750kHz
, smooth out the
LP
is 2200pF to
FREQUENCY
DETECTOR
DIGITAL
PHASE/
Auxiliary Winding Control Using SYNC/FCB Pin
The SYNC/FCB can be used as an auxiliary feedback to
provide a means of regulating a fl yback winding output.
When this pin drops below its ground-referenced 0.6V
threshold, continuous mode operation is forced.
During continuous mode, current fl ows 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
ratio is close to unity, the synchronous MOSFET may not be
on for a suffi cient amount of time to transfer power from
the output capacitor to the auxiliary load. Forced continu-
ous operation will support an auxiliary winding as long
as there is a suffi cient synchronous MOSFET duty factor.
The FCB input pin removes the requirement that power
must be drawn from the transformer 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 pulse-skipping operation
and halts switching due to a light primary load current, then
V
the FCB sets a minimum voltage V
2.4V
AUX
V
V
AUX
AUX MIN
will droop. An external resistor divider from V
PLLLPF
≅ (N + 1) V
(
R
LP
OSCILLATOR
)
=
37362 F09
0 6
C
LP
.
OUT
V
⎛
⎝ ⎜
1
+
R
R
AUX
6
5
⎞
⎠ ⎟
is normally set as shown
AUX(MIN)
:
IN
AUX
/ V
37362fb
OUT
to