LTC1439 Linear Technology, LTC1439 Datasheet - Page 19

• Manufacturer Part Number: LTC1439
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
C
driven from a low impedance such as a logic gate located
close to the pin. Any external attenuator used needs to be
referenced to SGND.
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
The low side of the filter needs to be connected to SGND.
The PLL LPF pin can be driven with external logic to obtain
a 1:1.9 frequency shift. The circuit shown in Figure 9 will
provide a frequency shift from f
V
V
Low-Battery Comparator
The LTC1438/LTC1439 have an on-chip low-battery com-
parator which can be used to sense a low-battery condi-
tion when implemented as shown in Figure 10. The resistor
divider R3/R4 sets the comparator trip point as follows:
The divided down voltage at the negative (–) input to the
comparator is compared to an internal 1.19V reference. A
20mV hysteresis is built in to assure rapid switching. The
output is an open drain MOSFET and requires a pull-up
resistor. This comparator is not active when both the
RUN/SS1 and RUN/SS2 pins are low. Refer to the LTC1538/
LTC1539 for a comparator which is active during shutdown.
The low side of the resistive divider needs to be connected to
SGND.
LP
PLLLPF
PLLLPF
V
holds the voltage. The LTC1439 PLLIN pin must be
LBITRIP
.
increases from OV to 2.4V. Do not exceed 2.4V on
Figure 9. Directly Driving PLL LPF Pin
PLL LPF
1 19 1
LP
.
and R
U
V
2.4V
MAX
18k
LP
LP
INFORMATION
R
R
U
=10k and C
4
3
determine how fast the loop
O
LP
3.3V OR 5V
to 1.9f
, R
W
LTC1435 • F09
LP
LP
O
is 0.01 F to 0.1 F.
smooth out the
as the voltage on
U
SFB1 Pin Operation
When the SFB1 pin drops below its ground referenced
1.19V threshold, continuous mode operation is forced. In
continuous mode, the large N-channel main and synchro-
nous switches are used regardless of the load on the main
output.
In addition to providing a logic input to force continuous
synchronous operation, the SFB1 pin provides a means to
regulate a flyback winding output. The use of a synchro-
nous switch removes the requirement that power must be
drawn from the inductor primary in order to extract power
from the auxiliary winding. With the loop in continuous
mode, the auxiliary output may be loaded without regard
to the primary output load. The SFB1 pin provides a way
to force continuous synchronous operation as needed by
the flyback winding.
The secondary output voltage is set by the turns ratio of
the transformer in conjunction with a pair of external
resistors returned to the SFB1 pin as shown in Figure 4a.
The secondary regulated voltage V
by:
where N is the turns ratio of the transformer, and V
the main output voltage sensed by Sense
Auxiliary Regulator/Comparator
The auxiliary regulator/comparator can be used as a
comparator or low dropout regulator (by adding an exter-
nal PNP pass device).
V
SEC
SGND
V
IN
N
Figure 10. Low-Battery Comparator
R4
R3
1
V
OUT
LBI
–
+
LTC1438/LTC1439
LTC1438/LTC1439
1.19V REFERENCE
1 19
.
V
1
SEC
R
R
in Figure 4a is given
6
5
LBO
–
1.
1438 F10
19
OUT
is