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參數(shù)資料
| 型號： | LTC3713 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Low Input Voltage, High Power, No RSENSE Synchronous Buck DC/DC Controller |
| 中文描述： | 低輸入電壓，高功率，無檢測電阻同步降壓DC / DC控制器 |
| 文件頁數(shù)： | 16/24頁 |
| 文件大?。?/td> | 325K |
| 代理商： | LTC3713 |
16
LTC3713
3713fa
converter. To ensure that the ripple current doesn’t exceed
a specified amount, the inductance can be chosen accord-
ing to the following equation:
L
V
V
V
I f
IN MIN
2
(
IN MAX
2
(
OUT BOOST
(
=
1
)
)
)
–
Diode D3 Selection
A Schottky diode is recommended for use in the boost
converter section. The Motorola MBR0520 is a very good
choice.
Boost Converter Output Capacitor
Because the LTC3713’s boost converter is internally com-
pensated, loop stability must be carefully considered when
choosing its output capacitor. Small, low cost tantalum
capacitors have some ESR, which aids stability. However,
ceramic capacitors are becoming more popular, having
attractive characteristics such as near-zero ESR, small size
and reasonable cost. Simply replacing a tantalum output
capacitor with a ceramic unit will decrease the phase margin,
in some cases to unacceptable levels. With the addition of
a phase-lead capacitor and isolating resistor, the boost
converter portion of the LTC3713 can be used success-
fully with ceramic output capacitors.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3713 circuits:
1. DC I
2
R losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode the average output current flows through L, but is
chopped between the top and bottom MOSFETs. If the two
MOSFETs have approximately the same R
DS(ON)
, then the
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
2
R
be estimated from:
C
SS
> C
OUT
V
OUT
R
SENSE
(10
–4
[F/V s])
Generally 0.1
μ
F is more than sufficient.
Overcurrent latchoff operation is not always needed or
desired. Load current is already limited during a short-
circuit by the current foldback circuitry and latchoff
operation can prove annoying during troubleshooting.
The feature can be overridden by adding a pull-up current
greater than 5
μ
A to the RUN/SS pin. The additional
current prevents the discharge of C
SS
during a fault and
also shortens the soft-start period. Using a resistor to V
IN
as shown in Figure 6a is simple, but slightly increases
shutdown current. Connecting a resistor to INTV
CC
as
shown in Figure 6b eliminates the additional shutdown
current, but requires a diode to isolate C
SS
. Any pull-up
network must be able to pull RUN/SS above the 4.2V
maximum threshold of the latchoff circuit and overcome
the 4
μ
A maximum discharge current.
INTV
CC
Supply
The 5V supply that powers the drivers and internal cir-
cuitry within the LTC3713 can be supplied by either an
internal P-channel low dropout regulator if V
IN
is greater
than 5V or the internal boost regulator if V
IN
is less than 5V.
The INTV
CC
pin can supply up to 50mA RMS and must be
bypassed to ground with a minimum of 4.7
μ
F tantalum or
other low ESR capacitor. Good bypassing is necessary to
supply the high transient currents required by the MOSFET
gate drivers. Applications using large MOSFETs with a
high input voltage and high frequency of operation may
cause the LTC3713 to exceed its maximum junction tem-
perature rating or RMS current rating. In continuous mode
operation, this current is I
GATECHG
= f(Q
g(TOP)
+ Q
g(BOT)
).
The junction temperature can be estimated from the
equations given in Note 2 of the Electrical Characteristics.
Inductor Selection for Boost Converter
For the boost converter, the inductance should be 4.7
μ
H
for input voltages less then 3.3V and 10
μ
H for inputs
above 3.3V. The inductor should have a saturation current
rating of approximately 0.5A or greater. A guide for select-
ing an inductor for the boost converter is to choose a ripple
current that is 40% of the current supplied by the boost
APPLICATIU
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