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參數(shù)資料
| 型號(hào)： | LT1952IGN |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Single Switch Synchronous Forward Controller |
| 中文描述： | 1 A SWITCHING CONTROLLER, 560 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 0.150 INCH, PLASTIC, SSOP-16 |
| 文件頁數(shù)： | 23/24頁 |
| 文件大小： | 252K |
| 代理商： | LT1952IGN |
LT1952
23
1952f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
APPLICATIU
Bus Converter: Optimum Output Voltage Tolerance
The Bus Converter applications shown on page 1 and in
Figure 16, provide semi-regulated isolated outputs with-
out the need for an optocoupler, optocoupler driver, refer-
ence or feedback network. The LT1952 Volt-Second clamp
adjusts switch duty cycle inversely proportional to input
voltage to provide an output voltage that is regulated against
input line variations. Some bus converters use a switch
duty cycle limit which causes output voltage variation of
typically
±
33% over a 2:1 input voltage range. The LT1952
typically provides a
±
10% output variation for the same
input variation. Typical output tolerance is further improved
for the LT1952 by inserting a resistor from the system
input voltage to the SS_MAXDC pin (Rx in Figure 19).
The LT1952 electrical specifications for the OUT Max Duty
Cycle Clamp show typical switch duty cycle to move from
72% to 33% for a 2x change of input voltage
(SS_MAXDC pin = 1.84V). Since output voltage regulation
follows V
IN
Duty Cycle, a switch duty cycle change of
72% to 36% (for a 2x input voltage change) provides
minimal output voltage variation for the LT1952 bus con-
verter. To achieve this, an SS_MAXDC pin voltage increase
of 1.09x (36/33) would be required at high input line. A
resistor Rx inserted between the SS_MAXDC pin and sys-
tem input voltage (Figure 19) increases SS_MAXDC volt-
age as input voltage increases, minimizing output voltage
variation over a 2:1 input voltage change.
The following steps determine values for Rx, R
T
and R
B
(1) Program switch duty cycle at minimum system input
voltage (V
S(MIN)
)
(a) R
T(1)
= 10k (minimum allowed to still guarantee soft-
start pull-down)
W
U
U
(b) Select switch duty cycle for the Bus Converter for a
given output voltage at V
S(MIN)
and calculate
SS_MAXDC voltage (SS1) (See Applications Informa-
tion “Programming Maximum Duty Cycle Clamp”)
(c) Calculate R
B(1)
= [SS1/(2.5 – SS1)] R
T(1)
(2) Calculate Rx
Rx = ([V
S(MAX) –
V
S(MIN)
]/[SS1 (X – 1)]) R
THEV(1)
R
THEV(1)
= R
B(1)
R
T(1)
/(R
B(1)
+ R
T(1)
), X = ideal duty
cycle (V
S(MAX)
)/actual duty cycle (V
S(MAX)
)
(3) The addition of Rx causes an increase in the original
programmed SS_MAXDC voltage SS1. A new value
for R
B(1)
should be calculated to provide a lower
SS_MAXDC voltage (SS2) to correct for this offset.
(a) SS2 = SS1 – [(V
S(MIN)
– SS1) R
THEV(1)
/Rx]
(b) R
B(2)
= [SS2/(2.5 – SS2)] R
T(1)
(4) The thevinin resistance R
THEV(1)
used to calculate Rx
should be re-established for R
T
and R
B
.
(a) R
B
(final value) = R
B(2)
(R
THEV(1)
/R
THEV(2)
)
(b) R
T
(final value) = R
T(1)
(R
THEV(1)
/R
THEV(2)
)
where R
THEV(2)
= R
B(2)
R
T(1)
/(R
B(2)
+ R
T(1)
)
Example:
For a Bus Converter running from 36V to 72V input,
V
S(MIN)
= 36V, V
S(MAX)
= 72V.
choose R
T(1)
= 10k, SS_MAXDC = SS1 = 1.84V (for 72%
duty cycle at V
S(MIN)
= 36V)
R
B(1)
= [1.84V/(2.5V – 1.84V)] 10k = 28k
R
THEV(1)
= [28k 10k/(28k + 10k)] = 7.4k
SS_MAXDC correction = 36%/33% = 1.09
Rx = [(72V – 36V)/(1.84 0.09)] 7.4k = 1.6M
SS2 = 1.84 – [(36V – 1.84) 7.4k/1.6M] = 1.682V
R
B(2)
= [1.682/(2.5 – 1.682)] 10k = 20.6k
R
THEV(2)
= [20.6k 10k/(20.6k + 10k)] = 6.7k
R
THEV(1)
/R
THEV(2)
= 7.4k/6.7k = 1.104
R
B
(final value) = 20.6k 1.104 = 22.7k (choose 22.6k)
R
T
(final value) = 10k 1.104 = 11k
Figure 19. Optimal Programming of Maximum Duty Cycle Clamp
for Bus Converter Applications (Adding Rx)
SYSTEM
INPUT VOLTAGE
VOLT-SECOND
CLAMP INPUT
VOLT-SECOND
CLAMP ADJUST INPUT
1952 F19
SD_V
SEC
SS_MAXDC
V
REF
LT1952
R1
Rx
R2
R
B
R
T
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