参数资料
| 型号： | LT1977IFE#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 16/24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
? 3 . 3 ? ( 125 μ A + 12 . 5 μ A + 0 . 5 μ A )
( 0 . 8 )
I IN ( AVG ) = 45 μ A + 5 μ A + ? ?
Example: For V OUT = 3.3V, V IN = 12V
? 12 ?
= 45 μ A + 5 μ A + 44 μ A = 99 μ A
V OUT
50mV/DIV
V SHDN
150
V OUT = 3.3V
T A = 25 ° C
2V/DIV
125
I SW
100
500mA/DIV
V IN = 12V
TIME (50ms/DIV)
1977 G16
75
50
25
0
V OUT = 3.3V
I Q = 15 μ A
Figure 6. Burst Mode with Shutdown Pin
To maximize high and low load current efficiency a fast
switching diode with low forward drop and low reverse
0
10
20 30 40
INPUT VOLTAGE (V)
50
60
1977 F05
leakage should be used. Low reverse leakage is critical to
maximize low current efficiency since its value over tem-
Figure 5. I Q vs V IN
During the sleep portion of the Burst Mode cycle, the V C
pin voltage is held just below the level needed for normal
operation to improve transient response. See the Typical
Performance Characteristics section for burst and tran-
sient response waveforms.
If a no load condition can be anticipated, the supply current
can be further reduced by cycling the SHDN pin at a rate
higher than the natural no load burst frequency. Figure 6
shows Burst Mode operation with the SHDN pin. V OUT
burst ripple is maintained while the average supply current
drops to 15 μ A. The PG pin will be active low during the
“on” portion of the SHDN waveform due to the C T capaci-
tor discharge when SHDN is taken low. See the Power
Good section for further information.
perature can potentially exceed the magnitude of the
LT1977 supply current. Low forward drop is critical for
high current efficiency since the loss is proportional to
forward drop.
These requirements result in the use of a Schottky type
diode. DC switching losses are minimized due to its low
forward voltage drop and AC behavior is benign due to its
lack of a significant reverse recovery time. Schottky diodes
are generally available with reverse voltage ratings of 60V
and even 100V and are price competitive with other types.
The effect of reverse leakage and forward drop on effi-
ciency for various Schottky diodes is shown in Table 5. As
can be seen these are conflicting parameters and the user
Table 5. Catch Diode Selection Criteria
I Q at 125 ° C EFFICIENCY
LEAKAGE
V IN =12V V IN =12V
CATCH DIODE
DIODE
V OUT = 3.3V
25 ° C 125 ° C
V F AT 1A
25 ° C
V OUT = 3.3 V OUT = 3.3V
I L = 0A I L = 1A
The catch diode carries load current during the SW off
time. The average diode current is therefore dependent on
the switch duty cycle. At high input to output voltage ratios
the diode conducts most of the time. As the ratio ap-
IR 10BQ100
Diodes Inc.
B260SMA
Diodes Inc.
0.0 μ A
0.1 μ A
0.2 μ A
59 μ A
242 μ A
440 μ A
0.72V
0.48V
0.45V
125 μ A
215 μ A
270 μ A
76.1%
80.4%
80.8%
proaches unity the diode conducts only a small fraction of
B360SMB
the time. The most stressful condition for the diode is
when the output is short circuited. Under this condition the
diode must safely handle I PEAK at maximum duty cycle.
IR
MBRS360TR
IR 30BQ100
1 μ A
0.5 μ A
1.81mA
225 μ A
0.42V
0.59V
821 μ A
206 μ A
81.4%
78.8%
1977fa
16
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