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参数资料
| 型号： | LT3500IMSE#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3.5 A SWITCHING REGULATOR, 2800 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | LEAD FREE, PLASTIC, MSOP-16 |
| 文件页数： | 13/28页 |
| 文件大小： | 318K |
| 代理商： | LT3500IMSE#PBF |
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LT3500
20
3500fc
If the synchronization signal changes between high and
low impedance states during power up (VOL,Hi-Z),connect
the synchronization circuitry to the LT3500 as shown in
the Typical Applications section. This will allow the LT3500
to start up with a switching frequency determined by the
equivalent resistance from the RT/SYNC pin to ground.
Shutdown and Undervoltage Lockout
Figure 8 shows how to add an undervoltage lockout (UVLO)
to the LT3500. Typically, UVLO is used in situations where
the input supply is current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
above the SHDN threshold. This can be used to set voltage
hysteresis of the UVLO using the following:
R1
=
V
H VL
2
μA
R2
=
0.76
VH 0.76
R1
+ 2.5μA
VH = Turn-on threshold
VL = Turn-off threshold
Example: switching should not start until the input is above
4.75V and is to stop if the input falls below 3.75V.
VH = 4.75V
VL = 3.75
R1
=
4.75
3.75
2
μA
~ 499k
R2
=
0.76
4.75
0.76
499k
+ 2.5μA
~ 71.5k
Keep the connections from the resistors to the SHDN
pin short and make sure that the interplane or surface
capacitance to switching nodes is minimized. If high re-
sistor values are used, the SHDN pin should be bypassed
with a 1nF capacitor to prevent coupling problems from
the switch node.
Soft-Start
The outputs of the LT3500 regulate to either the SS pin
voltage minus 100mV or an internally regulated 800mV,
whichever is lowest. A capacitor from the SS pin to ground
is charged by an internal 2.75μA current source resulting
in a linear output ramp from 0V to the regulated output
whose duration is given by:
t
RAMP =
C
SS 0.9V
2.75
μA
APPLICATIONS INFORMATION
Figure 8. Undervoltage Lockout
+
–
0.76V
2μA
2.5μA
R1
R2
C1
SHDN
VIN
3500 F08
An internal comparator will force the part into shutdown
below the minimum VIN of 2.8V. This feature can be
used to prevent excessive discharge of battery-operated
systems.
If an adjustable UVLO threshold is required, the SHDN
pin can be used. The threshold voltage of the SHDN pin
comparator is 0.76V. A 2.5μA internal current source de-
faults the open-pin condition to be operating (see Typical
Performance Characteristics). Current hysteresis is added
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