參數(shù)資料
型號(hào)： LTC1438XCG
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)： 20/32頁(yè)
文件大?。?/td> 596K
代理商： LTC1438XCG
20
LTC1438/LTC1439
APPLICATIO
S I
FOR
ATIO
U
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.
With the addition of an external PNP pass device, a linear
regulator capable of supplying up to 0.5A is created. As
shown in Figure 11a, the base of the external PNP con-
nects to the AUXDR pin together with a pull-up resistor.
The output voltage V
OAUX
at the collector of the external
PNP is sensed by the AUXFB pin.
The input voltage to the auxiliary regulator can be taken
from a secondary winding on the primary inductor as
shown in Figure 11a. In this application, the SFB1 pin
regulates the input voltage to the PNP regulator (see SFB1
Pin Operation) and should be set to approximately 1V to
2V above the required output voltage of the auxiliary
regulator. A Zener clamp diode may be required to keep the
secondary winding resultant output voltage under the 28V
AUXDR pin specification when the primary is heavily
loaded and the secondary is not.
The AUXFB pin is the feedback point of the regulator. An
internal resistor divider is available to provide a 12V output
by simply connecting AUXFB directly to the collector of the
external PNP. The internal resistive divider is switched in
when the voltage at AUXFB goes above 9.5V with 1V built-
in hysteresis. For other output voltages, an external resis-
tive divider is fed back to AUXFB as shown in Figure 11b.
The output voltage V
OAUX
is set as follows:
W
U
U
When used as a voltage comparator as shown in Figure
11c, the auxiliary block has a noninverting characteristic.
When AUXFB drops below 1.19V, the AUXDR pin will be
pulled low. A minimum current of 5
μ
A is required to pull
up the AUXDR pin to 5V when used as a comparator output
in order to counteract a 1.5
μ
A internal pull-down current
source.
V
V
R
R
OAUX
=
+
<
=
≥
1 19
.
1
8
7
8V AUXDR<8.5V
V
12V AUXDR
12V
OAUX
–
+
AUXON
AUXFB
ON/OFF
INPUT
V
PULL-UP
<
7.5V
AUXDR
OUTPUT
1438 F11c
1.19V REFERENCE
LTC1439
Figure 11c. Auxiliary Comparator Configuration
LTC1439
AUXDR
AUXFB
SFB1
AUXON
+
+
1438 F11a
V
SEC
SECONDARY
WINDING
1:N
ON/OFF
V
12V
R6
10
μ
F
R5
R6
R5
V
SEC
= 1.19V
> 13V
1 +
(
)
Figure 11a. 12V Output Auxiliary Regulator
Using Internal Feedback Resistors
Figure 11b. 5V Output Auxiliary Regulator Using
External Feedback Resistors
LTC1439
AUXDR
AUXFB
SFB1
AUXON
+
+
1438 F11b
V
SEC
SECONDARY
WINDING
1:N
ON/OFF
V
OAUX
R6
10
μ
F
R5
R8
R7
R6
R5
V
SEC
= 1.19V
> V
OAUX
1 +
(
)
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