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參數(shù)資料
| 型號： | LTC1437A |
| 廠商： | Linear Technology Corporation |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 抗輻射高效，5安培開關(guān)穩(wěn)壓器 |
| 文件頁數(shù)： | 11/28頁 |
| 文件大?。?/td> | 518K |
| 代理商： | LTC1437A |
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11
LTC1436A
LTC1436A-PLL/LTC1437A
OPERATIOU
Power-On Reset
The POR pin is an open drain output which pulls low when
the main regulator output voltage is out of regulation.
When the output voltage rises to within 7.5% of regula-
tion, a timer is started which releases POR after 2
16
(65536) oscillator cycles. In shutdown, the POR output is
pulled low.
(Refer to Functional Diagram)
Auxiliary Linear Regulator
The auxiliary linear regulator in the LTC1436A/LTC1437A
controls an external PNP transistor for operation up to
500mA. An internal AUXFB resistive divider set for 12V
operation is invoked when AUXDR pin is above 9.5V to
allow 12V VPP supplies to be easily implemented. When
AUXDR is below 8.5V an external feedback divider may be
used to set other output voltages. Taking the AUXON pin
low shuts down the auxiliary regulator providing a conve-
nient logic controlled power supply.
The AUX block can be used as a comparator having its
inverting input tied to the internal 1.19V reference. The
AUXDR pin is used as the output and requires an external
pull-up to a supply less than 8.5V in order to inhibit the
invoking of the internal resistive divider.
INTV
CC
/DRV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
of the other LTC1436A/LTC1437A circuitry is derived from
the INTV
CC
pin. The bottom MOSFET driver supply DRV
CC
pin is internally connected to INTV
CC
in the LTC1436A and
externally connected to INTV
CC
in the LTC1437A. When
the EXTV
CC
pin is left open, an internal 5V low dropout
regulator supplies INTV
CC
power. If EXTV
CC
is taken above
4.8V, the 5V regulator is turned off and an internal switch
is turned on to connect EXTV
CC
to INTV
CC
. This allows the
INTV
CC
power to be derived from a high efficiency external
source such as the output of the regulator itself or a
secondary winding, as described in the Applications Infor-
mation section.
APPLICATIO
S I
FOR
ATIO
U
The basic LTC1436A application circuit is shown in Figure
1, High Efficiency Step-Down Converter. External compo-
nent selection is driven by the load requirement, and
begins with the selection of R
SENSE
. Once R
SENSE
is
known, C
OSC
and L can be chosen. Next, the power
MOSFETs and D1 are selected. Finally, C
IN
and C
OUT
are
selected. The circuit shown in Figure 1 can be configured
for operation up to an input voltage of 28V (limited by the
external MOSFETs).
W
U
U
R
SENSE
Selection For Output Current
R
SENSE
is chosen based on the required output current.
The LTC1436A/LTC1437A current comparator has a maxi-
mum threshold of 150mV/R
SENSE
and an input common
mode range of SGND to INTV
CC
. The current comparator
threshold sets the peak of the inductor current, yielding a
maximum average output current I
MAX
equal to the peak
value less half the peak-to-peak ripple current
I
L
.
Allowing a margin for variations in the LTC1436A/
LTC1437A and external component values yields:
R
mV
SENSE
MAX
I
=
100
The LTC1436A/LTC1437A work well with R
SENSE
values
≥
0.005
.
C
OSC
Selection for Operating Frequency
The LTC1436A/LTC1437A use a constant frequency
architecture with the frequency determined by an external
oscillator capacitor C
OSC
. Each time the topside MOSFET
turns on, the voltage on C
OSC
is reset to ground. During the
on-time, C
OSC
is charged by a fixed current plus an
additional current which is proportional to the output
voltage of the phase detector V
PLLLPF
(LTC1436A-PLL/
LTC1437A). When the voltage on the capacitor reaches
1.19V, C
OSC
is reset to ground. The process then repeats.
The value of C
OSC
is calculated from the desired operating
frequency. Assuming the phase-locked loop has no exter-
nal oscillator input (V
PLLLPF
= 0V):
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