LTC1538-AUX Linear Technology, LTC1538-AUX Datasheet - Page 19

• Manufacturer Part Number: LTC1538-AUX
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
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 LTC1539 has an on-chip low battery comparator
which can be used to sense a low battery condition when
implemented as shown in Figure 10. This comparator is
active during shutdown allowing battery charge level
interrogation prior to and after powering up part or all of
the system. The resistor divider R3/R4 sets the compara-
tor 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 active when both the RUN/
SS1 and RUN/SS2 pins are low. The low side of the resistive
divider needs to be connected to SGND.
PLLLPF
PLLLPF
V
LBITRIP
.
increases from 0V to 2.4V. Do not exceed 2.4V on
SGND
V
IN
Figure 9. Directly Driving PLL LPF Pin
Figure 10. Low Battery Comparator
PLL LPF
1 19 1
LP
R4
R3
.
and R
U
V
LBI
2.4V
MAX
–
+
18k
LP
LP
INFORMATION
1.19V REFERENCE
R
R
U
= 10k and C
4
3
determine how fast the loop
LTC1539
O
3.3V OR 5V
to 1.9f
W
LTC1538 • F09
LP
O
LBO
is 0.01 F to 0.1 F.
as the voltage on
1538 F10
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
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).
When the voltage present at the AUXON pin is greater than
1.19V the regulator/comparator is on. The amplifier is
stable when operating as a low dropout regulator. This
same amplifier can be used as a comparator whose
inverting input is tied to the 1.19V reference.
The AUXDR pin is internally connected to an open drain
MOSFET which can sink up to 10mA. The voltage on
AUXDR determines whether or not an internal 12V resis-
tive divider is connected to AUXFB as described below. A
pull-up resistor is required on AUXDR and the voltage
must not exceed 28V.
V
SEC
N
LTC1538-AUX/LTC1539
1
V
OUT
1 19
.
V
SEC
1
in Figure 4a is given by:
R
R
6
5
–
1.
19
OUT
is