參數(shù)資料
型號(hào)： LT3845EFE#TR
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 600 kHz SWITCHING FREQ-MAX, PDSO16
封裝： 4.40 MM, PLASTIC, TSSOP-16
文件頁(yè)數(shù)： 7/24頁(yè)
文件大小： 263K
代理商： LT3845EFE#TR
LT3845
15
3845fb
APPLICATIONS INFORMATION
Choose the MOSFET VDSS specication to exceed the
maximum voltage across the drain to the source of the
MOSFET, which is VIN(MAX) plus any additional ringing
on the switch node. Ringing on the switch node can be
greatly reduced with good PCB layout and, if necessary,
an RC snubber.
In some applications, parasitic FET capacitances couple
the negative going switch node transient onto the bottom
gate drive pin of the LT3845, causing a negative voltage
in excess of the Absolute Maximum Rating to be imposed
on that pin. Connection of a catch Schottky diode from
this pin to ground will eliminate this effect. A 1A current
rating is typically sufcient of the diode.
The internal VCC regulator is capable of sourcing up to
40mA limiting the maximum total MOSFET gate charge,
QG, to 35mA/fSW. The QG vs VGS specication is typically
provided in the MOSFET data sheet. Use QG at VGS of 8V.
If VCC is back driven from an external supply, the MOSFET
drive current is not sourced from the internal regulator
of the LT3845 and the QG of the MOSFET is not limited
by the IC. However, note that the MOSFET drive current
is supplied by the internal regulator when the external
supply back driving VCC is not available such as during
start-up or short circuit.
The manufacturer’s maximum continuous drain current
specication should exceed the peak switch current,
IOUT(MAX) + ΔIL/2.
During the supply start-up, the gate drive levels are set by
the VCC voltage regulator, which is approximately 8V. Once
the supply is up and running, the VCC can be back driven
by an auxiliary supply such as VOUT. It is important not to
exceed the manufacturer’s maximum VGS specication.
A standard level threshold MOSFET typically has a VGS
maximum of 20V.
Input Capacitor Selection
A local input bypass capacitor is required for buck convert-
ers because the input current is pulsed with fast rise and
fall times. The input capacitor selection criteria are based
on the bulk capacitance and RMS current capability. The
bulk capacitance will determine the supply input ripple
voltage. The RMS current capability is used to prevent
overheating the capacitor.
The bulk capacitance is calculated based on maximum
input ripple,
ΔVIN:
C
IV
Vf
V
IN BULK
OUT MAX
OUT
IN
SW
IN MIN
()
=
Δ
ΔVIN is typically chosen at a level acceptable to the user.
100mV to 200mV is a good starting point. Aluminum elec-
trolytic capacitors are a good choice for high voltage, bulk
capacitance due to their high capacitance per unit area.
The capacitor’s RMS current is:
II
VV
V
CIN RMS
OUT
IN
OUT
IN
()
(–
)
()
=
2
If applicable, calculate it at the worst case condition,
VIN = 2VOUT. The RMS current rating of the capacitor
is specied by the manufacturer and should exceed the
calculated ICIN(RMS). Due to their low ESR (Equivalent
Series Resistance), ceramic capacitors are a good choice
for high voltage, high RMS current handling. Note that the
ripple current ratings from aluminum electrolytic capacitor
manufacturers are based on 2000 hours of life. This makes
it advisable to further derate the capacitor or to choose a
capacitor rated at a higher temperature than required.
The combination of aluminum electrolytic capacitors and
ceramic capacitors is an economical approach to meet-
ing the input capacitor requirements. The capacitor volt-
age rating must be rated greater than VIN(MAX). Multiple
capacitors may also be paralleled to meet size or height
requirements in the design. Locate the capacitor very close
to the MOSFET switch and use short, wide PCB traces to
minimize parasitic inductance.
Output Capacitor Selection
The output capacitance, COUT, selection is based on the
design’s output voltage ripple,
ΔVOUT and transient load
requirements.
ΔVOUT is a function of ΔIL and the COUT
ESR. It is calculated by:
Δ= Δ
+
V
I
ESR
fC
OUT
L
SW
OUT
(
)
1
8
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