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| 型号： | LT3478IFE-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 16/24页 |
| 文件大小： | 0K |
| 描述： | IC LED DRVR HP CONS CURR 16TSSOP |
| 标准包装： | 95 |
| 恒定电流： | 是 |
| 拓扑： | PWM，降压（降压），升压（升压） |
| 输出数： | 1 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 车载，背光 |
| 类型 - 次要： | 颜色，白色 LED |
| 频率： | 200kHz ~ 2.25MHz |
| 电源电压： | 2.8 V ~ 36 V |
| 输出电压： | 42V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 供应商设备封装： | 16-TSSOP-EP |
| 包装： | 管件 |
| 工作温度： | -40°C ~ 125°C |
| 产品目录页面： | 1339 (CN2011-ZH PDF) |
LT3478/LT3478-1
APPLICATIO S I FOR ATIO
(5) Higher output capacitor value improves PDR
(6) Choose the schottky diode (D2, Figure 9) for minimum
reverse leakage
See Typical Performance Characteristics graph “LED Cur-
rent vs PWM Duty Cycle”.
Soft-Start
To limit inrush current and output voltage overshoot dur-
ing startup/recovery from a fault condition, the LT3478/
LT3478-1 provide a soft-start pin SS. The SS pin is used
to program switch current ramp up timing using a ca-
pacitor to ground. The LT3478/LT3478-1 monitor system
parameters for the following faults: V IN <2.8V, ? S ? H ? D ? N <1.4,
inductor current >6A and boosted output voltage >OVP.
On detection of any of these faults, the LT3478/LT3478-1
stop switching immediately and a soft-start latch is set
causing the SS pin to be discharged (see Timing Diagram
for the SS pin in Figure 11). When all faults no longer ex-
ist and the SS pin has been discharged to at least 0.25V,
the soft-start latch is reset and an internal 12μA supply
charges the SS pin. A gradual ramp up of SS pin voltage
is equivalent to a ramp up of switch current limit until SS
exceeds V C .
The ramp rate of the SS pin is given by:
Δ V SS / Δ t = 12μA/C SS
SW
SS
To limit inductor current overshoot to <0.5A when SS
charges past the V C level required for loop control, the C SS
capacitor should be chosen using the following formula:
C SS(MIN) = C C (7.35 – 0.6(I LED ? V OUT /V S ))
Example: V S = 8V, V OUT = 16V, I LED = 1.05A, C C = 0.1μF,
C SS(MIN) = 0.1μF (7.35 – 0.6(1.05 ? 16/8))
= 0.612μF (choose 0.68μF).
High Inductor Current “Inrush” Protection
The LT3478/LT3478-1 provide an integrated resistor
between the V S and L pins to monitor inductor current
(Figure 1). During startup or “hotplugging” of the induc-
tor supply, it is possible for inductor currents to exceed
the maximum switch current limit. When inductor current
exceeds 6A, the LT3478/LT3478-1 protect the internal
power switch by turning it off and triggering a soft-start
latch. This protection prevents the switch from repetitively
turning on during excessive inductor currents by delay-
ing switching until the fault has been removed. To defeat
inductor current sensing the inductor supply should be
connected to the L pin and the V S pin left open. See details
in the Applications Information section “Soft-Start”.
LED Open Circuit Protection and Maximum PWM
Dimming Ratios
The LT3478/LT3478-1 LED drivers provide optimum pro-
tection from open LED faults by clamping the converter
output to a programmable overvoltage protection level
(OVP). In addition, the programmable OVP feature draws
zero current from the output during PWM = 0 to allow
FAULTS TRIGGERING
SOFT-START LATCH
WITH SW TURNED OFF
IMMEDIATELY:
V IN < 2.8V OR
0.65V (ACTIVE THRESHOLD)
0.25V (RESET THRESHOLD)
0.15V
higher PWM dimming ratios. This provides an advantage
over other LED driver applications which connect a resistor
divider directly from V OUT .
SHDN < 1.4V OR
V OUT > OVP OR
I (INDUCTOR) > 6A
SOFT-START
LATCH SET:
SOFT-START LATCH RESET:
SS < 0.25V AND
V IN > 2.8V AND
SHDN > 1.4V AND
V OUT < OVP AND
I (INDUCTOR) < 6A
3478 F11
An open LED fault occurs when the connection to the
LED(s) becomes broken or the LED(s) fails open. For an
LED driver using a step-up switching regulator, an open
circuit LED fault can cause the converter output to exceed
the voltage capabilities of the regulator’s power switch,
Figure 11. LT3478 Fault Detection and SS Pin Timing Diagram
causing permanent damage. When V OUT exceeds OVP, the
34781f
16
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