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參數(shù)資料
| 型號(hào)： | LT3437EDD |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 0.9 A SWITCHING REGULATOR, 240 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | 3 X 3 MM, 0.75 MM HEIGHT, PLASTIC, MO-229WEED-2, DFN-10 |
| 文件頁數(shù)： | 14/24頁 |
| 文件大?。?/td> | 523K |
| 代理商： | LT3437EDD |
LT3437
14
3437f
APPLICATIOU
A potential controllability problem could occur under
short-circuit conditions. If the power supply output is
short circuited, the feedback amplifier responds to the low
output voltage by raising the control voltage, V
C
, to its
peak current limit value. Ideally, the output switch would
be turned on, and then turned off as its current exceeded
the value indicated by V
C
. However, there is finite response
time involved in both the current comparator and turn-off
of the output switch. This results in a minimum on time
t
ON(MIN).
When combined with the large ratio of V
IN
to
(V
F
+ I R), the diode forward voltage plus inductor I R
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
W
U
U
f t
V
I R
V
ON
F
IN
≤
+
where:
f = switching frequency
t
ON
= switch on time
V
F
= diode forward voltage
V
IN
= Input voltage
I R = inductor I R voltage drop
If this condition is not observed, the current will not be
limited at I
PK
but will cycle-by-cycle ratchet up to some
higher value. Using the nominal LT3437 clock frequency
of 200kHz, a V
IN
of 40V and a (V
F
+ I R) of say 0.7V, the
maximum t
ON
to maintain control would be approximately
90ns, an unacceptably short time.
The solution to this dilemma is to slow down the oscillator
to allow the current in the inductor to drop to a sufficiently
low value such that the current does not continue to
ratchet higher. When the FB pin voltage is abnormally low,
thereby indicating some sort of short-circuit condition,
the oscillator frequency will be reduced. Oscillator fre-
quency is reduced by a factor of 10 when the FB pin voltage
is below 0.4V and increases linearly to its typical value of
200kHz at a FB voltage of 0.95V (see Typical Performance
Characteristics). These oscillator frequency reductions
during short-circuit conditions allow the LT3437 to main-
tain current control
SOFT-START
For applications where [V
IN
/(V
OUT
+ V
F
)] >10 or large input
surge currents cannot be tolerated, the LT3437 soft-start
feature should be used to control the output capacitor
charge rate during start-up, or during recovery from an
output short circuit, thereby adding additional control
over peak inductor current. The soft-start function limits
the switch current via the V
C
pin to maintain a constant
voltage ramp rate (dV/dt) at the output capacitor. A capaci-
tor (C1 in Figure 2) from the C
SS
pin to the regulated output
voltage determines the output voltage ramp rate. When
the current through the C
SS
capacitor exceeds the C
SS
threshold (I
CSS
), the voltage ramp of the output capacitor
is limited by reducing the V
C
pin voltage.
The C
SS
threshold
is proportional to the FB voltage (see Typical Performance
Characteristics) and is defeated for FB voltages greater
than 0.9V (typical). The output dV/dt can be approximated
by:
dV
dt
I
C
CSS
SS
=
but actual values will vary due to start-up load conditions,
compensation values and output capacitor selection.
V
1V/DIV
V
IN
= 12V
C
OUT
= 100
μ
F
I
LOAD
= 200mA
1ms/DIV
3437 F04
Figure 4. V
OUT
dV/dt
Burst Mode OPERATION
To enhance efficiency at light loads, the LT3437 automati-
cally switches to Burst Mode operation which keeps the
output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst
Mode operation, the LT3437 delivers short bursts of
C
SS
= GND
C
SS
= 0.1
μ
F
C
SS
= 0.01
μ
F
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