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參數(shù)資料
| 型號(hào)： | LM5010 |
| 廠商： | National Semiconductor Corporation |
| 元件分類(lèi)： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 抗輻射高效，5安培開(kāi)關(guān)穩(wěn)壓器 |
| 文件頁(yè)數(shù)： | 14/19頁(yè) |
| 文件大?。?/td> | 453K |
| 代理商： | LM5010 |
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Applications Information
(Continued)
(10)
The minimum ESR for C2 is then equal to:
If the capacitor used for C2 does not have sufficient ESR, R3
is added in series as shown in
Figure 1
. C2 should generally
be no smaller than 3.3 μF, although that is dependent on the
frequency and the allowable ripple amplitude at V
. Ex-
perimentation is usually necessary to determine the mini-
mum value for C2, as the nature of the load may require a
larger value. A load which creates significant transients re-
quires a larger value for C2 than a non-varying load.
D1:
The important parameters are reverse recovery time and
forward voltage drop. The reverse recovery time determines
how long the current surge lasts each time the buck switch is
turned on. The forward voltage drop is significant in the
event the output is short-circuited as it is mainly this diode’s
voltage (plus the voltage across the current limit sense re-
sistor) which forces the inductor current to decrease during
the off-time. For this reason, a higher voltage is better,
although that affects efficiency. A reverse recovery time of
)
30 ns, and a forward voltage drop of
)
0.75V are preferred.
The reverse leakage specification is important as that can
significantly affect efficiency. Other types of diodes may have
a lower forward voltage drop, but may have longer recovery
times, or greater reverse leakage. D1 should be rated for the
maximum V
, and for the peak current when in current limit
(I
PK
in
Figure 11
) which is equal to:
I
PK
= 1.5A + I
OR(max)
= 1.734A
where 1.5A is the maximum guaranteed current limit thresh-
old, and the maximum ripple current was previously calcu-
lated as 234 mAp-p. Note that this calculation is valid only
when R
CL
is not required.
C1:
Assuming the voltage supply feeding V
has a source
impedance greater than zero, this capacitor limits the ripple
voltage at V
while supplying most of the switch current
during the on-time. At maximum load current, when the buck
switch turns on, the current into V
IN
increases to the lower
peak of the output current waveform, ramps up to the peak
value, then drops to zero at turn-off. The average current into
V
during this on-time is the load current. For a worst case
calculation, C1 must supply this average load current during
the maximum on-time. The maximum on-time is calculated
using Equation 5, with a 25% tolerance added:
C1 is calculated from:
where I
is the load current, and
V is the allowable ripple
voltage at V
(1V for this example). Quality ceramic capaci-
tors with a low ESR should be used for C1. To allow for
capacitor tolerances and voltage effects, a 2.2 μF capacitor
will be used
C3:
The capacitor at the V
CC
pin provides not only noise
filtering and stability, but also prevents false triggering of the
V
CC
UVLO at the buck switch on/off transitions. For this
reason, C3 should be no smaller than 0.1 μF, and should be
a good quality, low ESR, ceramic capacitor. This capacitor
also determines the initial startup delay (t1 in
Figure 8
).
C4:
The recommended value for C4 is 0.022 μF. A high
quality ceramic capacitor with low ESR is recommended as
C4 supplies the surge current to charge the buck switch gate
at turn-on. A low ESR also ensures a complete recharge
during each off-time.
C5:
This capacitor suppresses transients and ringing due to
long lead inductance at V
. A low ESR, 0.1 μF ceramic chip
capacitor is recommended, located physically close to the
LM5010.
C6:
The capacitor at the SS pin determines the softstart
time, i.e. the time for the reference voltage at the regulation
comparator, and the output voltage, to reach their final value.
The time is determined from the following:
For a 5 ms softstart time, C6 calculates to 0.022 μF.
FINAL CIRCUIT
The final circuit is shown in
Figure 14
, and its performance is
shown in Figures 15 - 18.
L
www.national.com
14
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