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| 型号： | LTC1703IG#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 18/36页 |
| 文件大小： | 0K |
| 描述： | IC REG SW DUAL SYNC VID 28SSOP |
| 标准包装： | 2,000 |
| 应用： | 控制器，移动式 Intel Pentium? III |
| 输入电压： | 3 V ~ 7 V |
| 输出数： | 2 |
| 输出电压： | 0.9 V ~ 2 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-SSOP（0.209"，5.30mm 宽） |
| 供应商设备封装： | 28-SSOP |
| 包装： | 带卷 (TR) |
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LTC1703
APPLICATIO S I FOR ATIO
At the same time, the input supply needs to supply several
amps of current without excessive voltage drop. The input
supply must have regulation adequate to prevent sudden
load changes from causing the LTC1703 input voltage to
dip. In most typical applications where the LTC1703 is
generating a secondary low voltage logic supply, all of
these input conditions are met by the main system logic
supply when fortified with an input bypass capacitor.
INPUT BYPASS CAPACITOR
A typical LTC1703 circuit running from a 5V logic supply
might provide 1.6V at 10A at one of its outputs. 5V to 1.6V
implies a duty cycle of 32%, which means QT is on 32%
of each switching cycle. During QT’s on-time, the current
drawn from the input equals the load current and during
the rest of the cycle, the current drawn from the input is
near zero. This 0A to 10A, 32% duty cycle pulse train adds
up to 4.7A RMS at the input. At 550kHz, switching cycles
last about 1.8 μ s —most system logic supplies have no
hope of regulating output current with that kind of speed.
A local input bypass capacitor is required to make up the
difference and prevent the input supply from dropping
drastically when QT kicks on. This capacitor is usually
chosen for RMS ripple current capability and ESR as well
as value.
The input bypass capacitor in an LTC1703 circuit is
common to both channels. Consider our 10A example
case with the other side of the LTC1703 disabled. The input
The two sides of the LTC1703 run off a single master clock
and are wired 180 ° out of phase with each other to
significantly reduce the total capacitance/ESR needed at
the input. Assuming 100mV of ripple and 10A output
current, we needed an ESR of 0.01 ? and 4.7A ripple
current capability for one side. Now, assume both sides
are running simultaneously with identical loading. If the
two sides switched in phase, all the loading conditions
would double and we’d need enough capacitance for
9.4A RMS and 0.005 ? ESR. With the two sides out of
phase, the input current is 4.8A RMS —barely larger than
the single case (Figure 7)! The peak current deltas are still
only 10A, requiring the same 0.01 ? ESR rating. As long as
the capacitor we chose for the single side application can
support the slightly higher 4.8A RMS current, we can add
the second channel without changing the input capacitor
at all. As a general rule, an input bypass capacitor capable
of supporting the larger output current channel can sup-
port both channels running simultaneously (see the
2-Phase Operation section for more information). Details
on how to calculate the maximum RMS input current can
be found in Application Note 77.
Tantalum capacitors are a popular choice as input capaci-
tors for LTC1703 applications, but they deserve a special
caution here. Generic tantalum capacitors have a destruc-
tive failure mechanism when they are subjected to large
RMS currents (like those seen at the input of a LTC1703).
bypass capacitor gets exercised in three ways: its ESR
must be low enough to keep the initial drop as QT turns on
within reason (100mV or so); its RMS current capability
must be adequate to withstand the 4.7A RMS ripple current
at the input and the capacitance must be large enough to
maintain the input voltage until the input supply can make
10A
0
6.8A
0
32%
32%
68%
68%
QT CURRENT, SIDE 1 ONLY
(FOR 1-PHASE, 2 SIDES:
MULTIPLY CURRENT BY 2)
CURRENT IN C IN , SIDE 1 ONLY
I CIN = 4.66A RMS , (1-PHASE,
2 SIDES: I CIN = 9.3A RMS )
up the difference. Generally, a capacitor that meets the
first two parameters will have far more capacitance than is
–3.2A
32% 18% 32% 18%
required to keep capacitance-based droop under control.
In our example, we need 0.01 ? ESR to keep the input drop
under 100mV with a 10A current step and 4.7A RMS ripple
10A
0
QT1 CURRENT
QT2 CURRENT
BOTH SIDES EQUAL LOAD
2-PHASE OPERATION
current capacity to avoid overheating the capacitor. These
requirements can be met with multiple low ESR tantalum
3.6A
0
32% 18% 32% 18%
CURRENT IN C IN ,
BOTH SIDES EQUAL LOAD
or electrolytic capacitors in parallel, or with a large mono-
lithic ceramic capacitor.
–6.4A
I CIN = 4.8A RMS
1703 F07
Figure 7. Current Waveforms
1703fa
18
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