參數(shù)資料
型號(hào)： LTC1438CG-ADJ
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators
中文描述： 2 A DUAL SWITCHING CONTROLLER, 400 kHz SWITCHING FREQ-MAX, PDSO28
封裝： 0.209 INCH, PLASTIC, SSOP-28
文件頁(yè)數(shù)： 19/32頁(yè)
文件大?。?/td> 596K
代理商： LTC1438CG-ADJ
19
LTC1438/LTC1439
APPLICATIO
S I
N
FOR
ATIO
U
C
LP
holds the voltage. The LTC1439 PLLIN pin must be
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
LP
, R
LP
smooth out the
current pulses from the phase detector and provide a
stable input to the voltage-controlled oscillator. The filter
components C
LP
and R
LP
determine how fast the loop
acquires lock. Typically, R
LP
=10k and C
LP
is 0.01
μ
F to 0.1
μ
F.
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
O
to 1.9f
O
as the voltage on
V
PLLLPF
increases from OV to 2.4V. Do not exceed 2.4V on
V
PLLLPF
.
W
U
18k
3.3V OR 5V
PLL LPF
2.4V
MAX
LTC1435 F09
Figure 9. Directly Driving PLL LPF Pin
Figure 10. Low-Battery Comparator
–
+
LBI
V
IN
SGND
LBO
R4
R3
1438 F10
1.19V REFERENCE
LTC1438/LTC1439
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
SEC
in Figure 4a is given
by:
V
N
V
V
R
R
SEC
OUT
≈
+
(
)
>
+
1
1 19
.
1
6
5
where N is the turns ratio of the transformer, and V
OUT
is
the main output voltage sensed by Sense
–
1.
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).
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:
V
V
R
R
LBITRIP
=
+
1 19
.
1
4
3
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.
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