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參數(shù)資料
| 型號(hào)： | LTC3835EDHC-1#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A SWITCHING CONTROLLER, 580 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 5 X 3 MM, LEAD FREE, PLASTIC, MO-229WJED-1, DFN-16 |
| 文件頁數(shù)： | 5/28頁 |
| 文件大?。?/td> | 303K |
| 代理商： | LTC3835EDHC-1#TRPBF |
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LTC3835-1
13
38351fc
APPLICATIONS INFORMATION
The MOSFET power dissipations at maximum output
current are given by:
P
V
IR
V
I
MAIN
OUT
IN
MAX
DS ON
IN
MAX
=
() +
()
+
()
2
1
δ
()
2
11
()(
)
+
RC
VV
V
DR
MILLER
INTVCC
THMIN
TH
–
M
MIN
SYNC
IN
OUT
IN
MAX
f
P
VV
V
II
R
()
=
() +
()
–
2
δ
D
DS ON
()
where
δ is the temperature dependency of RDS(ON) and
RDR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. VTHMIN is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I2R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For VIN < 20V
the high current efciency generally improves with larger
MOSFETs, while for VIN > 20V the transition losses rapidly
increase to the point that the use of a higher RDS(ON) device
with lower CMILLER actually provides higher efciency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1 +
δ) is generally given for a MOSFET in the
form of a normalized RDS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diode D1 shown in Figure 8 conducts
during the dead-time between the conduction of the two
power MOSFETs. This prevents the body diode of the
bottom MOSFET from turning on, storing charge during
the dead-time and requiring a reverse recovery period that
could cost as much as 3% in efciency at high VIN. A 1A
to 3A Schottky is generally a good compromise for both
regions of operation due to the relatively small average
current. Larger diodes result in additional transition losses
due to their larger junction capacitance.
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (VOUT)/(VIN). To prevent
large voltage transients, a low ESR capacitor sized for the
maximum RMS current of one channel must be used. The
maximum RMS capacitor current is given by:
C RequiredI
IN
RMS ≈
()(
)
I
V
VV
V
MAX
IN
OUT
IN
OUT
–
12
/
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst-case condition is commonly
used for design because even signicant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 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. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3835-1, ceramic capacitors
can also be used for CIN. Always consult the manufacturer
if there is any question.
The selection of COUT is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satised, the capacitance is adequate for ltering. The
output ripple (
OUT) is approximated by:
ΔVI
ESR
fC
OUT
RIPPLE
OUT
≈+
1
8
where f is the operating frequency, COUT is the output
capacitance and IRIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since IRIPPLE increases with input voltage.
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