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参数资料
| 型号： | LTC3861EUHE#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 28/38页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 36-QFN |
| 标准包装： | 61 |
| 系列： | PolyPhase® |
| PWM 型： | 电压模式 |
| 输出数： | 2 |
| 频率 - 最大： | 2.1MHz |
| 占空比： | 91.5% |
| 电源电压： | 3 V ~ 5.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 36-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3861
APPLICATIONS INFORMATION
? f ?
? f ?
A ≈ 40log 1 + ? C ? – 20log 1 + ? C ? – 20log ( A MOD )
1 + LC ? ? 1 +
? f ? ? f P2(RES) f P2(RES) – f Z2(RES) ?
?
f C ? ?
f C
f Z2(RES)
R2 ? ?
f P2(RES) ?
?
? ?
≈ 20log ?
f LC
f C
+
? ? 1 + f
? ? ? ? 1 +
A MOD ≈
V IN(MAX) ? DC MAX
V RAMP
f LC =
f ESR =
f C = Crossover frequency =
f Z 1( ERR ) = f LC =
f Z 2 ( RES ) = =
f P 1 ( ERR ) = f ESR =
The RC network across the error amplifier and the feed-
forward components R3 and C3 introduce two pole-zero
pairs to obtain a phase boost at the system unity-gain
frequency, f C . In theory, the zeros and poles are placed
symmetrically around f C , and the spread between the zeros
and the poles is adjusted to give the desired phase boost
at f C . However, in practice, if the crossover frequency
is much higher than the LC double-pole frequency, this
method of frequency compensation normally generates
a phase dip within the unity bandwidth and creates some
concern regarding conditional stability.
If conditional stability is a concern, move the error ampli-
fier’s zero to a lower frequency to avoid excessive phase
dip. The following equations can be used to compute the
feedback compensation components value:
f SW = Switching frequency
1
2 π LC OUT
1
2 π R E SR C OUT
choose:
f SW
10
1
2 π R 2 C 1
f C 1
5 2 π ( R 1 + R 3 ) C 3
1
2 π R 2 ( C 1 // C 2 )
Required error amplifier gain at frequency f C :
2 2
? f LC ? ? f ESR ?
+ ?
R1
?
ESR f ESR – f LC f C ?
where AMOD is the modulator and line feedforward gain
and is equal to:
≈ 12V/ V
where DC MAX is the maximum duty cycle and V RAMP is
the line feedforward compensated PWM ramp voltage.
Once the value of resistor R1, poles and zeros location
have been decided, the value of R2, C1, C2, R3 and C3
can be obtained from the previous equations.
Compensating a switching power supply feedback loop
is a complex task. The applications shown in this data
sheet show typical values, optimized for the power
components shown. Though similar power compon-
ents should suffice, substantially changing even one
major power component may degrade performance
significantly. Stability also may depend on circuit board
layout. To verify the calculated component values, all
new circuit designs should be prototyped and tested
for stability.
f P 2 ( RES ) = 5 f C =
1
2 π R 3 C 3
28
For more information www.linear.com/LTC3861
3861fa
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