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参数资料
| 型号： | LTC3814IFE-5#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/30页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM CM 16-TSSOP |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 90% |
| 电源电压： | 4.5 V ~ 55 V |
| 降压： | 无 |
| 升压： | 是 |
| 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： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 带卷 (TR) |
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LTC3814-5
APPLICATIONS INFORMATION
When the controller is operating in continuous mode the
duty cycles for the top and bottom MOSFETs are given by:
Multiple MOSFETs can be used in parallel to lower
R DS(ON) and meet the current and thermal requirements
Main Switch Duty Cycle =
V OUT ? V IN
V OUT
if desired. The LTC3814-5 contains large low impedance
drivers capable of driving large gate capacitances without
signi?cantly slowing transition times. In fact, when driv-
Synchronous Switch Duty Cycle =
V IN
V OUT
ing MOSFETs with very low gate charge, it is sometimes
helpful to slow down the drivers by adding small gate
resistors (10 Ω or less) to reduce noise and EMI caused
The power dissipation for the main and synchronous
by the fast transitions.
MOSFETs at maximum output current are given by:
Operating Frequency
P MAIN = D MAX
I O(MAX)
1 D MAX
2
(
T )R DS(ON)
The choice of operating frequency is a tradeoff between
ef?ciency and component size. Low frequency operation
improves ef?ciency by reducing MOSFET switching losses
+
1
2
V OUT 2
I O(MAX)
1 D MAX
(R DR )(C MILLER )
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
?
1
INTV CC – V TH(IL)
+
1
V TH(IL)
(f)
The operating frequency of LTC3814-5 applications is
determined implicitly by the one-shot timer that controls
the on-time t OFF of the synchronous MOSFET switch.
(I O(MAX) ) 2 (
P SYNC =
t OFF = VOFF ( 76pF )
1
T ) R DS(0N)
1 D MAX
where ρ T is the temperature dependency of R DS(ON) , R DR
is the effective top driver resistance (approximately 2Ω at
V GS = V MILLER ). V TH(IL) is the data sheet speci?ed typical
gate threshold voltage speci?ed in the power MOSFET
data sheet at the speci?ed drain current. C MILLER is the
calculated capacitance using the gate charge curve from
The on-time is set by the current into the I OFF pin and the
voltage at the V OFF pin according to:
V
I IOFF
Tying a resistor R OFF from V OUT to the I OFF pin yields a syn-
chronous MOSFET on-time inversely proportional to V OUT .
This results in the following operating frequency and also
keeps frequency constant as V OUT ramps up at start-up:
the MOSFET data sheet and the technique described above.
Both MOSFETs have I 2 R losses while the bottom N-channel
f =
V IN
V VOFF ? R OFF (76pF)
(Hz)
equation includes an additional term for transition losses.
Both top and bottom MOSFET I 2 R losses are greatest at
lowest V IN , and the top MOSFET I 2 R losses also peak
during an overcurrent condition when it is on close to
100% of the period. For most LTC3814-5 applications,
the transition loss and I 2 R loss terms in the bottom
MOSFET are comparable, so best ef?ciency is obtained
by choosing a MOSFET that optimizes both R DS(ON) and
C MILLER . Since there is no transition loss term in the syn-
chronous MOSFET, however, optimal ef?ciency is obtained
by minimizing R DS(ON) —by using larger MOSFETs or
paralleling multiple MOSFETs.
The V OFF pin can be connected to INTV CC or ground or
can be connected to a resistive divider from V IN . The V OFF
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. Note, however, that
if the V OFF pin is connected to a constant voltage, the
operating frequency will be proportional to the input
voltage V IN . Figures 4a and 4b illustrate how R OFF relates
to switching frequency as a function of the input voltage
and V OFF voltage. To hold frequency constant for input
38145fc
13
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