参数资料
| 型号： | LT1977IFE#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/24页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 1.5A 16TSSOP |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1.2 V ~ 54 V |
| 输入电压： | 3.3 V ~ 60 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 500kHz |
| 电流 - 输出： | 1.5A |
| 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： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 16-TSSOP-EP |
LT1977
APPLICATIO S I FOR ATIO
must weigh the importance of each specification in choos-
ing the best diode for the application.
OPTIONAL
The use of so-called “ultrafast” recovery diodes is gener-
ally not recommended. When operating in continuous
mode, the reverse recovery time exhibited by “ultrafast”
V IN
V IN BOOST
LT1977
GND
SW
V OUT
diodes will result in a slingshot type effect. The power
internal switch will ramp up V IN current into the diode in an
attempt to get it to recover. Then, when the diode has
finally turned off, some tens of nanoseconds later, the V SW
node voltage ramps up at an extremely high dV/dt, per-
V BOOST – V SW = V OUT
V BOOST(MAX) = V IN + V OUT
(7a)
haps 5V to even 10V/ns! With real world lead inductances
the V SW node can easily overshoot the V IN rail. This can
V IN
V IN BOOST
LT1977
result in poor RFI behavior and, if the overshoot is severe
GND
SW
V OUT
enough, damage the IC itself.
V BOOST – V SW = V IN
BOOST PIN
For most applications the boost components are a 0.1 μ F
V BOOST(MAX) = 2V IN
(7b)
capacitor and a MMSD914 diode. The anode is typically
connected to the regulated output voltage to generate a
V IN
V IN BOOST
LT1977
V DC
voltage approximately V OUT above V IN to drive the output
stage (Figure 7a). However, the output stage discharges
GND
SW
D SS
V OUT
the boost capacitor during the on time of the switch. The
V BOOST – V SW = V DC
1977 F07
output driver requires at least 2.5V of headroom through-
out this period to keep the switch fully saturated. If the
output voltage is less than 3.3V it is recommended that an
alternate boost supply is used. The boost diode can be
connected to the input (Figure 7b) but care must be taken
to prevent the boost voltage (V BOOST = V IN ? 2) from
exceeding the BOOST pin absolute maximum rating. The
additional voltage across the switch driver also increases
power loss and reduces efficiency. If available, an inde-
pendent supply can be used to generate the required
BOOST voltage (Figure 7c). Tying BOOST to V IN or an
independent supply may reduce efficiency but it will re-
duce the minimum V IN required to start-up with light
loads. If the generated BOOST voltage dissipates too
much power at maximum load, the BOOST voltage the
LT1977 sees can be reduced by placing a Zener diode in
series with the BOOST diode (Figure 7a option).
A 0.1 μ F boost capacitor is recommended for most appli-
cations. Almost any type of film or ceramic capacitor is
suitable but the ESR should be <1 ? to ensure it can be fully
recharged during the off time of the switch. The capacitor
V BOOST(MAX) = V DC + V IN
(7c)
Figure 7. BOOST Pin Configurations
value is derived from worst-case conditions of 1800ns on
time, 40mA boost current and 0.7V discharge ripple. The
boost capacitor value could be reduced under less de-
manding conditions but this will not improve circuit opera-
tion or efficiency. Under low input voltage and low load
conditions a higher value capacitor will reduce discharge
ripple and improve start-up operation.
SHUTDOWN FUNCTION AND UNDERVOLTAGE
LOCKOUT
The SHDN pin on the LT1977 controls the operation of the
IC. When the voltage on the SHDN pin is below the 1.2V
shutdown threshold the LT1977 is placed in a “zero”
supply current state. Driving the SHDN pin above the
shutdown threshold enables normal operation. The SHDN
pin has an internal sink current of 3 μ A.
1977fa
17
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