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| 型号： | LT3972IMSE#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 10/24页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 3.5A 10MSOP |
| 标准包装： | 50 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.79 V ~ 30 V |
| 输入电压： | 3.6 V ~ 33 V |
| PWM 型： | 电流模式，Burst Mode? |
| 频率 - 开关： | 230kHz ~ 2.45MHz |
| 电流 - 输出： | 3.5A |
| 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： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽）裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 10-MSOP 裸露焊盘 |
LT3972
APPLICATIONS INFORMATION
V IN ( MAX ) =
V OUT + V D
f SW t ON ( MIN )
– V D + V SW
where I OUT(MAX) is the maximum output load current. To
guarantee suf?cient output current, peak inductor current
must be lower than the LT3972’s switch current limit (I LIM ).
L = ? OUT D ? ?? 1– OUT D ??
? f SW L ? ?
V IN ( MAX ) ?
V IN ( MIN ) =
V OUT + V D
where V IN(MAX) is the maximum operating input voltage,
V OUT is the output voltage, V D is the catch diode drop
(~0.5V), V SW is the internal switch drop (~0.5V at max
load), f SW is the switching frequency (set by R T ), and
t ON(MIN) is the minimum switch on-time (~100ns). Note that
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 33V are acceptable regardless of the
switching frequency. In this mode, the LT3972 may enter
pulse-skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode
the output voltage ripple and inductor current ripple will
be higher than in normal operation.
The minimum input voltage is determined by either the
LT3972’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
– V D + V SW
1– f SW t OFF ( MIN )
where V IN(MIN) is the minimum input voltage, and t OFF(MIN)
is the minimum switch off-time (150ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
The peak inductor current is:
I L(PEAK) = I OUT(MAX) + Δ I L /2
where I L(PEAK) is the peak inductor current, I OUT(MAX) is
the maximum output load current, and Δ I L is the inductor
ripple current. The LT3972’s switch current limit (I LIM ) is
5.5A at low duty cycles and decreases linearly to 4.5A at
DC = 0.8. The maximum output current is a function of
the inductor ripple current:
I OUT(MAX) = I LIM – Δ I L /2
Be sure to pick an inductor ripple current that provides
suf?cient maximum output current (I OUT(MAX) ).
The largest inductor ripple current occurs at the highest
V IN . To guarantee that the ripple current stays below the
speci?ed maximum, the inductor value should be chosen
according to the following equation:
? V + V ? ? V + V ?
Δ I
where V D is the voltage drop of the catch diode (~0.4V),
V IN(MAX) is the maximum input voltage, V OUT is the output
voltage, f SW is the switching frequency (set by RT), and
L is in the inductor value.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 5A. To keep the
ef?ciency high, the series resistance (DCR) should be less
than 0.1Ω, and the core material should be intended for
high frequency applications. Table 1 lists several vendors
and suitable types.
and switching frequency will determine the ripple current.
The ripple current ΔI L increases with higher V IN or V OUT
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
the ripple current is:
Δ I L = 0.4(I OUT(MAX) )
3972fa
10
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