參數(shù)資料
型號(hào)： L4992
廠商： STMICROELECTRONICS
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 300 kHz SWITCHING FREQ-MAX, PQFP32
封裝： TQFP-32
文件頁(yè)數(shù)： 8/26頁(yè)
文件大?。?/td> 296K
代理商： L4992
Output Capacitors
The output capacitor selection is based on the output voltage ripple requirements. This ripple is related to the
current ripple through the inductor and is almost entirely due to the ESR of the output capacitor. Therefore,
the goal is to achieve an ESR lower than a certain value, regardless of the actual capacitance value.
The maximum current ripple of the +5.1V section is:
IL5 = 2 (IL5pk - Iout5)
considering the values obtained in the paragraph "+5.1 V Inductor".
As for the +3.3V, the maximum ripple is given by:
IL3 = n
Iout12
Vin
3.3 +
3
4
3.3
(Vin 3.3)
fsw
L3p Vin
where VIN is Vinmin or Vinmax, as selected in the "+3.3V transformer" section.
Anyhow, the maximum ESR will be:
ESRx
≤
Vrppx
ILx
where the subscript x refers to either section.
In pulse-skipping operation, the capacitive component of the output ripple is comparable to the resistive
one, thus both should be considered:
Vrppx
(R) = 0.025 ESRX
Rsensex
Vrppx
(C) = 3.1 106 Lx
Coutx
1
Rsensex
2
1
Vin min
Vout
1
Vout
If specification on the output ripple under pulse-skipping condition is also given, Coutx and ESRX must
comply with it as well.
Further constraints on the minimum output capacitance can arise from specifications regarding the maxi-
mum undershoot,
V-out, or overshoot, V+out, due to a step-load change Iout:
Cout
>
L
Iout
2
Vout
(V
in min
Dmax Vout)
; Cout
>
L
Iout
2
Vout
+ V
out
whichever is greater, and where Dmax is the maximum duty cycle and the quantities are relevant either to
the +3.3V or +5.1V section.
High performance capacitors should be employed to reduce the capacitance needed for a given ESR, to
avoid paralleling several parts with a considerable waste of space. Although excellent electrolytic capaci-
tors are available, OS-CON or tantalums may be preferred especially if very compact design is required,
or in case of surface mount assemblies. Multilayer ceramic capacitors with extremely low ESR are now
available, but they have a large spread of the capacitance value, so they should be paralleled with an-
other more stable, high-ESR capacitor.
Miscellaneous components
The feedback loop has virtually unlimited bandwidth, thus a filter is necessary to make the system insen-
sitive to the switching frequency ripple and, in general, to prevent noise from disturbing the correct op-
eration of the error summing comparator. Anyway, the cut-off frequency of this filter can be very high, so
that line and load transient response is extremely fast. This filter is a simple R-C type where resistance
and capacitance can be chosen for a typical 3dB cut-off frequency of 60 kHz.
As to the bootstrap diodes, even though small signal p-n diodes might be effectively used, it is preferable
to employ low-power Schottky rectifiers, since that increase slightly the gate drive voltage, in favour of ef-
ficiency. The bootstrap capacitor can be a 100nF film capacitor.
The soft-start capacitors determine the time during which the current limitation circuit moves gradually the
setpoint from zero up to 100 mV in order to limit the current inflow at start-up. This ramp lasts approximately
1 ms per nF of soft-start capacitance (10 to 100 nF typical values), but the actual time necessary to the out-
put voltage to reach the steady-state value depends on the load current and the output filter capacitance.
There are some critical points of the IC that may require by-pass capacitors to prevent noise from dis-
DESIGN PROCEDURE (continued)
L4992
16/26
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
L4993 制造商:STMICROELECTRONICS 制造商全稱:STMicroelectronics 功能描述:Low drop voltage regulator
L4993D 功能描述:低壓差穩(wěn)壓器 - LDO Low drop voltage regulator RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20
L4993DTR 功能描述:低壓差穩(wěn)壓器 - LDO Low drop voltage regulator RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20
L4993MD 功能描述:低壓差穩(wěn)壓器 - LDO Low drop voltage regulator RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20
L4993MDTR 功能描述:低壓差穩(wěn)壓器 - LDO 5V 150mA LW DROP VLT REGULATOR RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20
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