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參數(shù)資料
| 型號： | LTC3708 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | No RSENSE Low EMI, Synchronous DC/DC Controller with Output Tracking |
| 中文描述： | 無檢測電阻低EMI，同步DC / DC控制器的輸出跟蹤 |
| 文件頁數(shù)： | 18/28頁 |
| 文件大小： | 372K |
| 代理商： | LTC3708 |
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18
LTC3808
3808f
For coincident tracking (V
OUT
= V
X
during start-up),
R
TA
= R
A
, R
TB
= R
B
V
X
should always be greater than V
OUT
when using the
tracking function of TRACK/SS pin.
The internal current source (1
μ
A), which is for external
soft-start, will cause a tracking error at V
OUT
. For example,
if a 59k resistor is chosen for R
TA
, the R
TA
current will be
about 10
μ
A (600mV/59k). In this case, the 1
μ
A internal
current source will cause about 10% (1
μ
A/10
μ
A 100%)
tracking error, which is about 60mV (600mV 10%)
referred to V
FB
. This is acceptable for most applications. If
a better tracking accuracy is required, the value of R
TA
should be reduced.
Table 1 summarizes the different states in which the
TRACK/SS can be used.
Table 1. The States of the TRACK/SS Pin
TRACK/SS Pin
Capacitor C
SS
V
IN
Resistor Divider
FREQUENCY
External Soft-Start
Internal Soft-Start
V
OUT
Tracking an External Voltage V
X
Phase-Locked Loop and Frequency Synchronization
The LTC3808 has a phase-locked loop (PLL) comprised of
an internal voltage-controlled oscillator (VCO) and a phase
detector. This allows the turn-on of the external P-channel
MOSFET to be locked to the rising edge of an external clock
signal applied to the SYNC/MODE pin. The phase detector
is an edge sensitive digital type that provides zero degrees
phase shift between the external and internal oscillators.
This type of phase detector does not exhibit false lock to
harmonics of the external clock.
The output of the phase detector is a pair of complemen-
tary current sources that charge or discharge the external
filter network connected to the PLLLPF pin. The relation-
ship between the voltage on the PLLLPF pin and operating
frequency, when there is a clock signal applied to SYNC/
MODE, is shown in Figure 6 and specified in the electrical
characteristics table. Note that the LTC3808 can only be
synchronized to an external clock whose frequency is
within range of the LTC3808’s internal VCO, which is
APPLICATIU
W
U
U
TIME
(5b) Coincident Tracking
V
X
V
OUT
O
TIME
3808 F05b,c
(5c) Ratiometric Tracking
V
X
V
OUT
O
Figure 5b and 5c. Two Different Modes of Output Voltage Tracking
PLLLPF PIN VOLTAGE (V)
0.2
0
F
0.7
1.2
1.7
3808 F06
2.2
200
400
600
800
1000
1200
Figure 6. Relationship Between Oscillator Frequency
and Voltage at the PLLLPF Pin When Synchronizing to
an External Clock
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