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| 型号： | LTC3704IMS#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 19/28页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR INV PWM CM 10-MSOP |
| 标准包装： | 50 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 97% |
| 电源电压： | 2.5 V ~ 36 V |
| 降压： | 无 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 是 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽） |
| 包装： | 管件 |
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LTC3704
APPLICATIO S I FOR ATIO
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount packages. In the case of
tantalum, it is critical that the capacitors have been surge
tested for use in switching power supplies. An excellent
choice is AVX TPS series of surface mount tantalum. Also,
ceramic capacitors are now available with extremely low
ESR, ESL and high ripple current ratings.
Input Capacitor Selection
The input voltage source impedance determines the size of
the input capacitor, which is typically in the range of 10 μ F
to 100 μ F. A low ESR capacitor is recommended, although
it is not as critical as for the output capacitor.
The RMS input capacitor ripple current for a positive-to-
negative converter is:
causes the inductor current to quickly decay to zero.
However, because Δ I L is small, it takes multiple cycles for
the current to ramp back up to I BURST(PEAK) . During this
inductor charging interval, the output capacitor must
supply the load current and a significant droop in the
output voltage can occur. Generally, it is a good idea to
choose a value of inductor Δ I L between 20% and 40% of
I IN(MAX) . The alternative is to either increase the value of
the output capacitor or disable Burst Mode operation
using the MODE/SYNC pin.
Burst Mode operation can be defeated by connecting the
MODE/SYNC pin to a high logic-level voltage (either with
a control input or by connecting this pin to INTV CC ). In this
mode, the burst clamp is removed, and the chip can
operate at constant frequency from continuous conduc-
tion mode (CCM) at full load, down into deep discontinu-
I RMS ( CIN ) =
1
12
?
V IN(MIN)
L 1 ? f
? D MAX
ous conduction mode (DCM) at light load. Prior to skip-
ping pulses at very light load (i.e., < 5-10% of full load), the
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to the
input of the converter and solid tantalum capacitors can
fail catastrophically under these conditions. Be sure to
specify surge-tested capacitors!
Burst Mode Operation and Considerations
The choice of MOSFET R DS(ON) and inductor value also
determines the load current at which the LTC3704 enters
Burst Mode operation. When bursting, the controller clamps
the peak inductor current to approximately:
controller will operate with a minimum switch on-time in
DCM. Pulse skipping prevents a loss of control of the
output at very light loads and reduces output voltage
ripple.
Checking Transient Response
The regulator loop response can be verified by looking at
the load transient response. Switching regulators gener-
ally take several cycles to respond to an instantaneous
step in resistive load current. When the load step occurs,
V O immediately shifts by an amount equal to ( Δ I LOAD )(ESR),
and then C O begins to charge or discharge (depending on
I BURST ( PEAK ) =
30 mV
R DS ( ON )
the direction of the load step) as shown in Figure 14. The
which represents about 20% of the maximum 150mV
SENSE pin voltage. The corresponding average current
depends upon the amount of ripple current. Lower induc-
tor values (higher Δ I L ) will reduce the load current at which
V OUT (AC)
100mV/DIV
Burst Mode operations begins, since it is the peak current
that is being clamped.
I OUT (DC)
1A/DIV
2A
0.5A
The output voltage ripple can increase during Burst Mode
operation if Δ I L is substantially less than I BURST . This can
occur if the input voltage is very low or if a very large
V IN = 5V
V OUT = –5V
250 μ s/DIV
3704 F14
inductor is chosen. At high duty cycles, a skipped cycle
Figure 14. Load Step Response for the Circuit in Figure 1.
3704fb
19
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