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| 型号： | LT3508IUF#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 9/26页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 1.4A DL 24QFN |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 2 |
| 输出电压： | 0.8 V ~ 35.3 V |
| 输入电压： | 3.7 V ~ 36 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 920kHz ~ 1.06MHz |
| 电流 - 输出： | 1.4A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 24-WFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 24-QFN 裸露焊盘（4x4） |
LT3508
APPLICATIONS INFORMATION
R1 = R2 ? OUT – 1 ?
V IN(MIN) =
– 0.4V + 0.4V = 3.8V
V OUT + V F
V IN – V SW + V F
V IN(PS) =
– V F + V SW
V IN(PS) =
– 0.4V + 0.4V = 36V
Setting the Output Voltage
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
? V ?
? 0.8V ?
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Minimum Operating Voltage
The minimum operating voltage is determined either by
the LT3508’s undervoltage lockout or by its maximum duty
cycle. If V IN1 and V IN2 are tied together, the undervoltage
lockout is at 3.7V or below. If the two inputs are used
separately, then V IN1 has an undervoltage lockout of 3.7V
or below and V IN2 has an undervoltage lockout of 3V or
below. Because the internal supply runs off V IN1 , chan-
nel 2 will not operate unless V IN1 > 3.7V. The duty cycle
is the fraction of time that the internal switch is on and is
determined by the input and output voltages:
DC =
Unlike many ?xed frequency regulators, the LT3508 can
extend its duty cycle by turning on for multiple cycles.
The LT3508 will not switch off at the end of each clock
cycle if there is suf?cient voltage across the boost capaci-
tor (C3 in Figure 1). Eventually, the voltage on the boost
capacitor falls and requires refreshing. Circuitry detects
this condition and forces the switch to turn off, allowing
the inductor current to charge up the boost capacitor. This
places a limitation on the maximum duty cycle as follows:
where V F is the forward voltage drop of the catch diode
(~0.4V) and V SW is the voltage drop of the internal switch
(~0.4V at maximum load).
Example: I SW = 1.5A and I BOOST = 50mA, V OUT = 3.3V,
β SW = 1.5A/50mA = 30, DC MAX = 1/(1+1/30) = 96%:
3.3V + 0.4V
96%
Maximum Operating Voltage
The maximum operating voltage is determined by the
Absolute Maximum Ratings of the V IN and BOOST pins,
and by the minimum duty cycle:
DC MIN = t ON(MIN) ? f
where t ON(MIN) is equal to 130ns (for T J > 125°C t ON(MIN)
is equal to 150ns) and f is the switching frequency.
Running at a lower switching frequency allows a lower
minimum duty cycle. The maximum input voltage before
pulse skipping occurs depends on the output voltage and
the minimum duty cycle:
V OUT + V F
DC MIN
Example: f = 790kHz, V OUT = 3.3V, DC MIN = 130ns ? 790kHz
= 0.103:
3.3V + 0.4V
0.103
The LT3508 will regulate the output current at input voltages
greater than V IN(PS) . For example, an application with an
output voltage of 1.8V and switching frequency of 1.5MHz
has a V IN(PS) of 11.3V, as shown in Figure 2. Figure 3 shows
DC MAX =
1 +
1
1
β SW
operation at 18V. Output ripple and peak inductor current
have signi?cantly increased. Exceeding V IN(PS) is safe if
the output is in regulation, if the external components have
adequate ratings to handle the peak conditions and if the
where β SW is equal to the SW pin current divided by the
BOOST pin current as shown in the Typical Performance
Characteristics section. This leads to a minimum input
voltage of:
peak inductor current does not exceed 3.2A. A saturating
inductor may further reduce performance. Do not exceed
V IN(PS) during start-up or overload conditions (for outputs
greater than 5V, use V OUT = 5V to calculate V IN(PS) ). For
V IN(MIN) =
V OUT + V F
DC MAX
– V F + V SW
operation above 20V in pulse skipping mode, program
the switching frequency to 1.1MHz or less.
3508fd
9
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