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參數(shù)資料
| 型號(hào)： | LTC4269IDKD-2#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 560 kHz SWITCHING FREQ-MAX, PDSO32 |
| 封裝： | 7 X 4 MM, PLASTIC, M0-229, DFN-32 |
| 文件頁數(shù)： | 21/34頁 |
| 文件大?。?/td> | 383K |
| 代理商： | LTC4269IDKD-2#TR |
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LTC4269-2
42692fb
applicaTions inForMaTion
The time for SS_MAXDC to fall to a given voltage can be
approximated as:
SS_MAXDC(tFALL)=
(CSS/IDIS) [SS_MAXDC(DC) – VSS(MIN)]
where:
IDIS = net discharge current on CSS
CSS = capacitor value at SS_MAXDC pin
SS_MAXDC(DC) = programmed DC voltage
VSS(MIN) = minimum SS_MAXDC voltage before
recharge
IDIS 8e–4 + (VREF – VSS(MIN))[(1/2RB) – (1/RT)]
For faults arising from (1) and (2):
VREF = 100mV.
For a fault arising from (3):
VREF = 2.5V.
SS_MAXDC(DC) = VREF[RB/(RT + RB)]
VSS(MIN) = SS_MAXDC reset threshold = 0.45V
(if fault removed before tFALL)
Example
For an overcurrent fault (OC > 100mV), VREF = 2.5V,
RT = 35.7k, RB = 100k, CSS = 0.1F and assume
VSS(MIN) = 0.45V,
IDIS 8e–4 + (2.5 – 0.45)[( 100k) – (1/35.7k)]
= 8e–4 + (2.05)(–0.23e–4) = 7.5e–4
SS_MAXDC(DC) = 1.84V
SS_MAXDC(tFALL)
= (1e–7/7.5e–4) (1.84 – 0.45)=1.85e–4s
IftheOCfaultisnotremovedbefore185sthenSS_MAXDC
will continue to fall past 0.45V towards a new VSS(MIN).
The typical VOL for SS_MAXDC at 150A is 0.2V.
SS_MAXDC Charge Timing
When all faults are removed and the SS_MAXDC pin
has fallen to its reset threshold of 0.45V or lower, the
SS_MAXDC pin will be released and allowed to charge.
SS_MAXDC will rise until it settles at its programmed DC
voltage—setting the maximum switch duty cycle clamp.
The calculation of charging time for the SS_MAXDC pin
between any two voltage levels can be approximated as
an RC charging waveform using the model shown in
Figure 16.
TheabilitytopredictSS_MAXDCrisetimebetweenanytwo
voltages allows prediction of several key timing periods:
(1) No Switching Period (time from SS_MAXDC(DC) to
VSS(MIN) + time from VSS(MIN) to VSS(ACTIVE))
(2) Converter Output Rise Time (time from VSS(ACTIVE) to
VSS(REG); VSS(REG) is the level of SS_MAXDC where
maximum duty cycle clamp equals the natural duty
cycle of the switch)
(3) Time For Maximum Duty Cycle Clamp within X% of
Target Value
The time for SS_MAXDC to charge to a given voltage VSS
is found by re-arranging:
VSS(t) = SS_MAXDC(DC) (1 – e(–t/RC))
to give,
t = RC (–1) ln(1 – VSS/SS_MAXDC(DC))
where,
VSS = SS_MAXDC voltage at time t
SS_MAXDC(DC) = programmed DC voltage setting
maximum duty cycle clamp = VREF(RB/(RT + RB)
R = RCHARGE (Figure 16) = RT RB/(RT + RB)
C = CSS (Figure 16)
Example (1) No Switching Period
The period of no switching for the converter, when a soft-
start event has occurred, depends on how far SS_MAXDC
can fall before recharging occurs and how long a fault ex-
ists. It will be assumed that a fault triggering soft-start is
removed before SS_MAXDC can reach its reset threshold
(0.45V).
No Switching Period = tDISCHARGE + tCHARGE
tDISCHARGE = discharge time from SS_MAXDC(DC) to
0.45V
tCHARGE = charge time from 0.45V to VSS(ACTIVE)
tDISCHARGE was already calculated earlier as 185s.
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