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参数资料
| 型号： | LTC3879EUD#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/28页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16-QFN |
| 标准包装： | 121 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 2MHz |
| 占空比： | 90% |
| 电源电压： | 4 V ~ 38 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 ~ 85°C |
| 封装/外壳： | 16-WFQFN 裸露焊盘 |
| 包装： | 管件 |
| 产品目录页面： | 1336 (CN2011-ZH PDF) |
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LTC3879
APPLICATIONS INFORMATION
Figure 3 shows how R ON relates to switching frequency
for several common output voltages.
When designing for pseudo ?xed frequency, there is sys-
4
3
tematic error because the I ON pin voltage is approximately
0.7V, not zero. This causes the I ON current to be inversely
proportional to (V IN – 0.7V) and not V IN . The I ON current
error increases as V IN decreases. To correct this error, an
additional resistor R ON2 can be connected from the I ON
pin to the 5.3V INTV CC supply.
2
1
DROPOUT
REGION
R ON 2 =
5 . 3 V – 0 . 7 V
0 . 7 V
R ON
0
0
0.25 0.50 0.75
DUTY CYCLE (V OUT /V IN )
1
3879 F04
1000
V OUT = 1.5V
V OUT = 3.3V
V OUT = 12V
Figure 4. Maximum Switching Frequency vs Duty Cycle
Likewise, the maximum frequency of operation is deter-
mined by the ?xed on-time, t ON , and the minimum off-time,
t OFF(MIN) . The ?xed on-time is determined by dividing the
duty factor by the nominal frequency of operation:
V IN OP
V OUT = 5V
f MAX =
V OUT
? f
1
+ t OFF ( MIN )
??[ Hz ]
100
100
1000
R ON (kΩ)
10000
3879 F03
The LTC3879 is a PFM (pulse frequency mode) regula-
tor where pulse density is modulated, not pulse width.
Δ I L = ? OUT ? ? 1 – OUT ?
Figure 3. Switching Frequency vs R ON
Minimum Off-Time and Dropout Operation
The minimum off-time, t OFF(MIN) , is the shortest time
required for the LTC3879 to turn on the bottom MOSFET,
trip the current comparator and then turn off the bottom
MOSFET. This time is typically about 220ns. The minimum
off-time limit imposes a maximum duty cycle of t ON /
(t ON + t OFF(MIN) ). If the maximum duty cycle is reached,
due to a drooping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
Consequently, frequency increases with a load step and
decreases with a load release. The steady-state operating
frequency, f OP , should be set suf?ciently below f MAX to
allow for device tolerances and transient response.
Inductor Value Calculation
Given the desired input and output voltages, the induc-
tor value and operation frequency determine the ripple
current:
? V ? ? V ?
? f OP ? L ? ? V IN ?
V IN ( MIN ) = V OUT
t ON + t OFF (MIN)
t ON
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest ef?ciency operation is obtained at low
A plot of maximum duty cycle vs. frequency is shown in
Figure 4.
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, ef?ciency and operating frequency.
3879f
13
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