参数资料
型号: LT3689EUD#TRPBF
厂商: Linear Technology
文件页数: 18/34页
文件大小: 0K
描述: IC REG BUCK ADJ 0.7A 16QFN
标准包装: 2,500
类型: 降压(降压)
输出类型: 可调式
输出数: 1
输出电压: 0.8 V ~ 34.2 V
输入电压: 3.6 V ~ 36 V
PWM 型: 电流模式,Burst Mode?
频率 - 开关: 500kHz ~ 2MHz
电流 - 输出: 700mA
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: 16-WFQFN 裸露焊盘
包装: 带卷 (TR)
供应商设备封装: 16-QFN-EP(3x3)
LT3689/LT3689-5
APPLICATIONS INFORMATION
Table 4. Diode Vendors
V R I AVE V F AT I AVE
PART NUMBER (V) (A) (mV)
On Semiconductor
MBRM120E 20 1 530
MBRM140 40 1 550
Diodes Inc.
B120 20 1 500
B130 30 1 500
B140 40 1 500
B0540W 40 0.5 510
B140HB 40 1 530
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3689 due to their piezoelectric
nature. When in Burst Mode operation, the LT3689’s
switching frequency depends on the load current, and at
very light loads the LT3689 can excite the ceramic capaci-
tor at audio frequencies, generating audible noise. Since
the LT3689 operates at a lower current limit during Burst
Mode operation, the noise is typically very quiet. If this
noise is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
Frequency Compensation
The LT3689 uses current mode control to regulate the
output, which simplifies loop compensation. In particular,
the LT3689 does not require the ESR of the output capaci-
tor for stability, allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size. Figure 3
shows an equivalent circuit for the LT3689 control loop.
The error amp is a transconductance amplifier with finite
output impedance. The power section, consisting of the
modulator, power switch and inductor, is modeled as a
transconductance amplifier generating an output current
proportional to the voltage at the V C node. Note that the
output capacitor, C1, integrates this current, and that the
capacitor on the V C node (C C ) integrates the error ampli-
fier output current, resulting in two poles in the loop. R C
provides a zero. With the recommended output capacitor,
the loop crossover occurs above the R C C C zero. This simple
model works well as long as the value of the inductor is
not too high and the loop crossover frequency is much
lower than the switching frequency. With a larger ceramic
capacitor (very low ESR), crossover may be lower and a
phase lead capacitor (C PL ) across the feedback divider may
improve the phase margin and transient response. Large
electrolytic capacitors may have an ESR large enough to
create an additional zero, and the phase lead may not be
necessary.
LT3689
0.7V
g m =
1.2A/V
CURRENT MODE
POWER STAGE
OUT
FB
R1
C PL
V C
R C
37k
C C
100pF
3M
g m =
300μA/V
ERROR
AMPLIFIER
800mV
R2
+
ESR
C1
C1
GND
3689 F03
Figure 3. Model for the Loop Response
3689fe
18
For more information www.linear.com/LT3689
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