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参数资料
| 型号： | LTC3778EF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 15/24页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 20TSSOP |
| 标准包装： | 74 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 电源电压： | 4 V ~ 36 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： | 20-TSSOP（0.173"，4.40mm 宽） |
| 包装： | 管件 |
| 产品目录页面： | 1336 (CN2011-ZH PDF) |
LTC3778
APPLICATIO S I FOR ATIO
EXTV CC Connection
The EXTV CC pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTV CC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV CC
pin to INTV CC . INTV CC power is supplied from EXTV CC until
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV CC pin and ensure that EXTV CC ≤ V IN . The follow-
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3778 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC3778 into a low quiescent current shutdown (I Q <
30 μ A). Releasing the pin allows an internal 1.2 μ A current
source to charge up the external timing capacitor C SS . If
RUN/SS has been pulled all the way to ground, there is a
delay before starting of about:
ing list summarizes the possible connections for EXTV CC :
1. EXTV CC grounded. INTV CC is always powered from the
internal 5V regulator.
t DELAY =
1 . 5 V
1 . 2 μ A
C SS = ( 1 . 3 s / μ F ) C SS
2. EXTV CC connected to an external supply. A high effi-
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV CC connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
will start-up using the internal linear regulator until the
boosted output supply is available.
External Gate Drive Buffers
The LTC3778 drivers are adequate for driving up to about
60nC into MOSFET switches with RMS currents of 50mA.
Applications with larger MOSFET switches or operating at
frequencies requiring greater RMS currents will benefit
from using external gate drive buffers such as the LTC1693.
Alternately, the external buffer circuit shown in Figure 5
can be used. Note that the bipolar devices reduce the
signal swing at the MOSFET gate, and benefit from an
increased EXTV CC voltage of about 6V.
When the voltage on RUN/SS reaches 1.5V, the LTC3778
begins operating with a clamp on I TH of approximately
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I TH
is raised until its full 2.4V range is available. This takes an
additional 1.3s/ μ F, during which the load current is folded
back until the output reaches 50% of its final value. The pin
can be driven from logic as shown in Figure 6. Diode D1
reduces the start delay while allowing C SS to charge up
slowly for the soft-start function.
After the controller has been started and given adequate
time to charge up the output capacitor, C SS is used as a
short-circuit timer. After the RUN/SS pin charges above
4V, if the output voltage falls below 75% of its regulated
value, then a short-circuit fault is assumed. A 1.8 μ A cur-
rent then begins discharging C SS . If the fault condition
persists until the RUN/SS pin drops to 3.5V, then the con-
troller turns off both power MOSFETs, shutting down the
converter permanently. The RUN/SS pin must be actively
pulled down to ground in order to restart operation.
The overcurrent protection timer requires that the soft-
TG
10 ?
BOOST
Q1
FMMT619
GATE
OF M1
Q2
FMMT720
BG
10 ?
DRV CC
Q3
FMMT619
GATE
OF M2
Q4
FMMT720
start timing capacitor C SS be made large enough to guar-
antee that the output is in regulation by the time C SS has
reached the 4V threshold. In general, this will depend upon
the size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor can
be estimated from:
SW PGND
Figure 5. Optional External Gate Driver
3778 F05
C SS > C OUT V OUT R SENSE (10 – 4 [F/V s])
Generally 0.1 μ F is more than sufficient.
3778f
15
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