參數(shù)資料
型號: LT3689IMSE-5#TRPBF
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: 1.95 A SWITCHING REGULATOR, 2160 kHz SWITCHING FREQ-MAX, PDSO16
封裝: LEAD FREE, PLASTIC, MSOP-16
文件頁數(shù): 6/32頁
文件大?。?/td> 2188K
代理商: LT3689IMSE-5#TRPBF
LT3689/LT3689-5
14
3689fd
Maximum Operating Voltage
The maximum input voltage for LT3689 applications
depends on switching frequency, the absolute maximum
ratings of the VIN and BST pins, and by the minimum
duty cycle (DCMIN). The LT3689 can operate from input
voltages up to 36V, and safely withstand input transient
voltages up to 60V. Note that while VIN > 38V (typical),
the LT3689 will stop switching, allowing the output to fall
out of regulation.
DCMIN = tON(MIN) fSW
where tON(MIN) is equal to 130ns (for TJ > 125°C tON(MIN)
is equal to 150ns) and fSW is the switching frequency.
Running at a lower switching frequency allows a lower
minimum duty cycle. The maximum input voltage before
pulse-skipping occurs depends on the output voltage and
the minimum duty cycle:
VIN(PS) =
VOUT + VD
DCMIN
– VD + VSW
Example: fSW = 790kHz, VOUT = 3.3V,
DCMIN = 130ns 790kHz = 0.103
VIN(PS) =
3.3V + 0.4V
0.103
– 0.4V + 0.4V = 36V
The LT3689 will regulate the output current at input volt-
ages greater than VIN(PS). For example, an application
with an output voltage of 1.8V and switching frequency
of 1.5MHz has a VIN(PS) of 11.3V, as shown in Figure 1.
Figure 2 shows operation at 24V. Output ripple and peak
inductor current have significantly increased. A saturating
inductor may further reduce performance. For input volt-
ages over 30V, there are restrictions on the inductor size
and saturation rating. See the Inductor Selection section
formoredetails.Inpulse-skippingmode,theLT3689skips
switchingpulsestomaintainoutputregulation.Above38V
(typical) switching will stop. Transients of up to 60V are
acceptable, regardless of switching frequency.
Minimum Operating Voltage Range
The minimum input voltage is determined either by the
LT3689’s minimum operating voltage of ~3.4V or by its
maximum duty cycle. The duty cycle is the fraction of
time that the internal switch is on and is determined by
the input and output voltages:
DC =
VOUT + VD
VIN – VSW + VD
Unlike many fixed frequency regulators, the LT3689 can
extend its duty cycle by remaining on for multiple cycles.
TheLT3689willnotswitchoffattheendofeachclockcycle
if there is sufficient voltage across the boost capacitor (C3
in the Block Diagram). Eventually, the voltage on the boost
capacitor falls and requires refreshing. Circuitry detects
APPLICATIONS INFORMATION
Figure 1. Operation Below VIN(PS). VIN = 10V,
VOUT = 1.8V and fSW = 1.5MHz
Figure 2. Operation Above VIN(PS). VIN = 24V,
VOUT = 1.8V and fSW = 1.5MHz. Output Ripple
and Peak Inductor Current Increase
5s/DIV
VOUT
50mV/DIV
(AC)
IL
0.5A/DIV
3689 F01
5s/DIV
VOUT
50mV/DIV
(AC)
IL
0.5A/DIV
3689 F02
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