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參數(shù)資料
| 型號(hào)： | DC1145B |
| 廠商： | Linear Technology |
| 文件頁數(shù)： | 16/32頁 |
| 文件大?。?/td> | 0K |
| 描述： | EVAL BOARD FOR LTC4267 |
| 設(shè)計(jì)資源： | DC1145B Design Files |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 主要目的： | 電源管理，以太網(wǎng)供電（POE） |
| 嵌入式： | 否 |
| 已用 IC / 零件： | LTC4267 |
| 已供物品： | 板 |
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LTC4267
23
4267fc
The shunt regulator can sink up to 5mA through the PVCC
pin to PGND. The values of RSTART and CPVCC must be
selected for the application to withstand the worst-case
load conditions and drop on PVCC, ensuring that the PVCC
turn-off threshold is not reached. CPVCC should be sized
sufciently to handle the switching current needed to drive
NGATE while maintaining minimum switching voltage.
actual current needed to power the LTC4267 switching
regulator goes through Q1 and PVCC sources current on
an “as-needed” basis. The static current is then limited
only to the current through RB and D1.
APPLICATIO S I FOR ATIO
WU
UU
Figure 12. Powering the LTC4267 Switching
Regulator via the Shunt Regulator
Figure 13. Powering the LTC4267 Switching
Regulator with an External Preregulator
VPORTP
PVCC
PGND
POUT
VPORTN
LTC4267
–48
FROM
PSE
RSTART
CPVCC
+
–
PGND
4267 F14
External Preregulator
The circuit in Figure 13 shows a third way to power the
LTC4267 switching regulator circuit. An external series
preregulator consists of a series pass transistor Q1, zener
diode D1, and a bias resistor RB. The preregulator holds
PVCC at 7.6V nominal, well above the maximum rated PVCC
turn-off threshold of 6.8V. Resistor RSTART momentarily
charges the PVCC node up to the PVCC turn-on threshold,
enabling the switching regulator. The voltage on CPVCC
begins to decline as the switching regulator draws its
normal supply current, which exceeds the delivery of
RSTART. After some time, the output voltage approaches
the desired value. By this time, the pass transistor Q1
catches the declining voltage on the PVCC pin, and provides
virtually all the supply current required by the LTC4267
switching regulator. CPVCC should be sized sufciently to
handle the switching current needed to drive NGATE while
maintaining minimum switching voltage.
The external preregulator has improved efciency over
the simple resistor-shunt regulator method mentioned
previously. RB can be selected so that it provides a small
current necessary to maintain the zener diode voltage and
the maximum possible base current Q1 will encounter. The
VPORTP
PVCC
PGND
POUT
VPORTN
LTC4267
–48
FROM
PSE
RSTART
CPVCC
+
–
PGND
Q1
D1
8.2V
RB
4267 F15
Compensating the Main Loop
In an isolated topology, the compensation point is typically
chosen by the components congured around the external
error amplier. Shown in Figure 14, a series RC network
is connected from the compare voltage of the error am-
plier to the error amplier output. In PD designs where
transient load response is not critical, replace RZ with a
short. The product of R2 and CC should be sufciently large
to ensure stability. When fast settling transient response
is critical, introduce a zero set by RZCC. The PD designer
must ensure that the faster settling response of the output
voltage does not compromise loop stability.
In a nonisolated design, the LTC4267 incorporates an
internal error amplier where the ITH/RUN pin serves as
a compensation point. In a similar manner, a series RC
network can be connected from ITH/RUN to PGND as
shown in Figure 15. CC and RZ are chosen for optimum
load and line transient response.
Figure 14. Main Loop Compensation for an Isolated Design
R1
R2
CC
RZ
TO OPTO-
ISOLATOR
4267 F14
VOUT
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