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LT3799
OPERATION
gate, a current comparator, a low output current oscillator
and a master latch, which will be explained in the follow-
ing sections. The part also features a sample-and-hold
to detect open LED conditions, along with a FAULT pin. A
comparator is used to detect discontinuous conduction
mode (DCM) with a cap connected to the third winding.
The part features a 1.9A gate driver.
The LT3799 employs a micropower hysteretic start-up
feature to allow the part to work at any combination of
input and output voltages. In the Block Diagram, R3 is used
to stand off the high voltage supply voltage. The internal
LDO starts to supply current to the INTV CC when V IN is
above 23V. The V IN and INTV CC capacitors are charged by
the current from R3. When V IN exceeds 23V and INTV CC is
in regulation at 10V, the part will began to charge the CT
pin with 10μA. Once the CT pin reaches 340mV, switching
begins. The V IN pin has 10.7V of hysteresis to allow for
plenty of flexibility with the input and output capacitor
values. The third winding provides power to V IN when its
voltage is higher than the V IN voltage. A voltage shunt is
provided for fault protection and can sink up to 15mA of
current when V IN is over 25V.
During a typical cycle, the gate driver turns the external
MOSFET on and a current flows through the primary
winding. This current increases at a rate proportional
to the input voltage and inversely proportional to the
magnetizing inductance of the transformer. The control
loop determines the maximum current and the current
comparator turns the switch off when the current level
is reached. When the switch turns off, the energy in the
core of the transformer flows out the secondary winding
through the output diode, D1. This current decreases at a
rate proportional to the output voltage. When the current
decreases to zero, the output diode turns off and voltage
across the secondary winding starts to oscillate from the
parasitic capacitance and the magnetizing inductance of
the transformer. Since all windings have the same voltage
across them, the third winding rings too. The capacitor
connected to the DCM pin, C1, trips the comparator, A2,
which serves as a dv/dt detector, when the ringing occurs.
This timing information is used to calculate the output
current (description to follow). The dv/dt detector waits
for the ringing waveform to reach its minimum value and
then the switch turns back on. This switching behavior is
similar to zero volt switching and minimizes the amount of
energy lost when the switch is turned back on, improving
efficiency as much as 5%. Since this part operates on the
edge of continuous conduction mode and discontinuous
conduction mode, this operating mode is called critical
conduction mode (or boundary conduction mode).
Primary-Side Current Control Loop
The CTRL1/CTRL2/CTRL3 pins control the output current
of the flyback controller. To simplify the loop, assume
the V IN_SENSE pin is held at a constant voltage above
1V, eliminating the multiplier from the control loop. The
error amplifier, A5, is configured as an integrator with
the external capacitor, C6. The COMP + node voltage is
converted to a current into the multiplier with the V/I
converter, A6. Since A7’s output is constant, the output
of the multiplier is proportional to A6 and can be ignored.
The output of the multiplier controls the peak current with
its connection to the current comparator, A1. The output
of the multiplier is also connected to the transmission
gate, SW1. The transmission gate, SW1, turns on when
the secondary current flows to the output capacitor. This
is called the flyback period (when the output diode D1 is
on). The current through the 1M resistor gets integrated
by A5. The lowest CTRL input is equal to the negative input
of A5 in steady state.
A current output regulator normally uses a sense resistor
in series with the output current and uses a feedback loop
to control the peak current of the switching converter. In
this isolated case the output current information is not
available, so instead the LT3799 calculates it using the
information available on the primary side of the trans-
former. The output current may be calculated by taking the
average of the output diode current. As shown in Figure
1, the diode current is a triangle waveform with a base of
the flyback time and a height of the peak secondary wind-
ing current. In a flyback topology, the secondary winding
current is N times the primary winding current, where
N is the primary to secondary winding ratio. Instead of
taking the area of the triangle, think of it as a pulse width
modulation (PWM) waveform. During the flyback time,
the average current is half the peak secondary winding
3799fa
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LT3799IMSE#PBF
制造商:Linear Technology 功能描述:IC LED DRVR MSOP-16 制造商:Linear Technology 功能描述:IC, LED DRVR, MSOP-16
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功能描述:IC LED DRIVER TRIAC DIM 16-MSOP RoHS:是 类别:集成电路 (IC) >> PMIC - LED 驱动器 系列:- 标准包装:6,000 系列:- 恒定电流:- 恒定电压:- 拓扑:开路漏极，PWM 输出数:4 内部驱动器:是 类型 - 主要:LED 闪烁器 类型 - 次要:- 频率:400kHz 电源电压:2.3 V ~ 5.5 V 输出电压:- 安装类型:表面贴装 封装/外壳:8-VFDFN 裸露焊盘 供应商设备封装:8-HVSON 包装:带卷 (TR) 工作温度:-40°C ~ 85°C 其它名称:935286881118PCA9553TK/02-TPCA9553TK/02-T-ND
LT3799IMSE-1#PBF
功能描述:IC LED CTLR ISO ACT PFC 16MSOP RoHS:是 类别:集成电路 (IC) >> PMIC - LED 驱动器 系列:- 标准包装:6,000 系列:- 恒定电流:- 恒定电压:- 拓扑:开路漏极，PWM 输出数:4 内部驱动器:是 类型 - 主要:LED 闪烁器 类型 - 次要:- 频率:400kHz 电源电压:2.3 V ~ 5.5 V 输出电压:- 安装类型:表面贴装 封装/外壳:8-VFDFN 裸露焊盘 供应商设备封装:8-HVSON 包装:带卷 (TR) 工作温度:-40°C ~ 85°C 其它名称:935286881118PCA9553TK/02-TPCA9553TK/02-T-ND
LT3799IMSE-1#TRPBF
功能描述:IC LED CTLR ISO ACT PFC 16MSOP RoHS:是 类别:集成电路 (IC) >> PMIC - LED 驱动器 系列:- 标准包装:6,000 系列:- 恒定电流:- 恒定电压:- 拓扑:开路漏极，PWM 输出数:4 内部驱动器:是 类型 - 主要:LED 闪烁器 类型 - 次要:- 频率:400kHz 电源电压:2.3 V ~ 5.5 V 输出电压:- 安装类型:表面贴装 封装/外壳:8-VFDFN 裸露焊盘 供应商设备封装:8-HVSON 包装:带卷 (TR) 工作温度:-40°C ~ 85°C 其它名称:935286881118PCA9553TK/02-TPCA9553TK/02-T-ND
LT3799IMSEPBF
制造商:LINER 制造商全称:Linear Technology 功能描述:Offline Isolated Flyback LED Controller with Active PFC
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制造商:LINER 制造商全称:Linear Technology 功能描述:High Voltage, Current Mode Switching Regulator Controller