参数资料
型号: LTC3608IWKG#TRPBF
厂商: Linear Technology
文件页数: 11/26页
文件大小: 0K
描述: IC REG BUCK SYNC ADJ 8A 52QFN
特色产品: 8A, 18V Synchronous Step-Down Regulator
标准包装: 2,000
类型: 降压(降压)
输出类型: 可调式
输出数: 1
输出电压: 0.6 V ~ 17.8 V
输入电压: 4 V ~ 18 V
PWM 型: 电流模式
电流 - 输出: 8A
同步整流器:
工作温度: -40°C ~ 125°C
安装类型: 表面贴装
封装/外壳: 52-VFQFN 裸露焊盘
包装: 带卷 (TR)
供应商设备封装: 52-QFN(7x8)裸露焊盘
LTC3608
APPLICATIONS INFORMATION
t ON =
V VON
I ON = IN
V OUT
V VON
R ON (10pF)
The basic LTC3608 application circuit is shown on the
front page of this data sheet. External component selection
is primarily determined by the maximum load current.
The LTC3608 uses the on-resistance of the synchronous
power MOSFET for determining the inductor current. The
desired amount of ripple current and operating frequency
also determines the inductor value. Finally, C IN is selected
for its ability to handle the large RMS current into the
converter and C OUT is chosen with low enough ESR to meet
the output voltage ripple and transient specification.
V ON  and PGOOD
The LTC3608 has an open-drain PGOOD output that
indicates when the output voltage is within ±10% of the
regulation point. The LTC3608 also has a V ON pin that
allows the on-time to be adjusted. Tying the V ON pin high
results in lower values for R ON which is useful in high V OUT
applications. The V ON pin also provides a means to adjust
the on-time to maintain constant frequency operation in
applications where V OUT changes and to correct minor
frequency shifts with changes in load current.
V RNG  Pin and I LIMIT  Adjust
The V RNG pin is used to adjust the maximum inductor
valley current, which in turn determines the maximum
average output current that the LTC3608 can deliver. The
maximum output current is given by:
I OUT(MAX) = I VALLEY(MAX) + 1/2 Δ I L
The I VALLEY(MAX) is shown in the figure “Maximum Valley
Current Limit vs V RNG Voltage” in the Typical Performance
Characteristics.
An external resistor divider from INTV CC can be used to
set the voltage on the V RNG pin from 0.5V to 1V, or it can
be simply tied to ground force a default value equivalent
to 0.7V. When setting current limit ensure that the junc-
tion temperature does not exceed the maximum rating of
125°C. Do not float the V RNG pin.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3608 applications is de-
termined implicitly by the one-shot timer that controls the
on-time, t ON , of the top MOSFET switch. The on-time is
set by the current into the I ON pin and the voltage at the
V ON pin according to:
(10pF)
I ION
Tying a resistor R ON from V IN to the I ON pin yields an
on-time inversely proportional to V IN . The current out of
the I ON pin is
V
R ON
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
f = [H Z ]
To hold frequency constant during output voltage changes,
tie the V ON pin to V OUT or to a resistive divider from V OUT
when V OUT > 2.4V. The V ON pin has internal clamps that
limit its input to the one-shot timer. If the pin is tied below
0.7V, the input to the one-shot is clamped at 0.7V. Similarly,
if the pin is tied above 2.4V, the input is clamped at 2.4V.
In high V OUT applications, tying V ON to INTV CC so that the
comparator input is 2.4V results in a lower value for R ON .
Figures 1a and 1b show how R ON relates to switching
frequency for several common output voltages.
3608fc
  
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