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參數(shù)資料
| 型號： | TK11830 |
| 廠商： | TOKO Inc. |
| 元件分類： | DC/DC變換器 |
| 英文描述： | POSITIVE-TO-NEGATIVE DC-DC CONVERTER |
| 中文描述： | 正到負(fù)DC - DC轉(zhuǎn)換器 |
| 文件頁數(shù)： | 11/13頁 |
| 文件大?。?/td> | 85K |
| 代理商： | TK11830 |
January 1999 TOKO, Inc.
Page 11
TK11830
ILPK(MAX)
IC
IL
tON
tOFF
ti
ILPK(MAX)
-ILOAD
ILOAD
Q1-
Q+
Q2-
t
C
I
INTERMITTENT OSCILLATION
When the ripple voltage applied to the feedback pin is large
and C
FB
is small, the power transistor switch drive is large
and the output voltage exceeds the desired value. This
causes the oscillator to stop for a period of t
i
. When the
ripple voltage is large and the power transistor is driven at
maximum capacity, a current up to I
LPK(MAX)
goes through
the inductor.
Note: t
ON
/t
OFF
= (|V
OUT
| + V
F
) / (V
IN
- V
SAT
)
t
ON
= [I
LPK(MAX)
/ (V
IN
- V
SAT
)] x L
t
OFF
= [I
LPK(MAX)
/ (|V
OUT
| + V
F
)] x L
Since the charge of the capacitor is equivalent to the
discharge (
Q
+
=
Q
1
-
+
Q
2
-
):
I
LPK(MAX)
= 2 x I
LOAD
x [(t
ON
/ t
OFF
) + 1] + 2 x I
LOAD
x
(t
i
/
t
OFF
)
t
i
= ([I
LPK(MAX)
/ (2 x I
LOAD
)] x t
OFF
) - (t
ON
+ t
OFF
)
f = 1 / (t
ON
+ t
OFF
+ t
i
)
When load current increases, t
i
becomes shorter.
As in the case above, if the load current is too small, the
power transistor becomes overdriven and intermittent
oscillation will occur.
PACKAGE POWER DISSIPATION
The internal thermal protection circuit will operate when T
j
is approximately 150
°
C. When thermal protection operates,
the power transistor switch will cycle between on and off to
keep T
j
≤
150
°
C. Thermal resistance O
j
a is determined by
mounting. The package power dissipation curve on a
printed circuit board is estimated as follows:
When Pin 4 is connected to GND (Power transistor switch
is at maximum conductance), all input power is dissipated
by the IC at T
A
= room temperature. In this state T
j
goes up
to 150
°
C and thermal protection operates. Input power is
defined as P
IN
= V
IN
x <I
IN
>, where <I
IN
> is the average of
input current. From T
j
= O
j
a x P + T
A
and T
j
= 150
°
C.
P = P
IN
, T
A
= Room temp., O
j
a can be found. The power
dissipation curve shows the effect of mounting on thermal
characteristics.
P
LOSS
, must be within this curve. The efficiency, E (%), is
the ratio between input and output power when the dc-dc
converter is operating.
P
LOSS
= P
IN
- P
OUT
= P
OUT
x [(100 / E) - 1]
= |V
OUT
| x I
LOAD
x [(100 / E) - 1]
APPLICATION INFORMATION (CONT.)
IIN
t
<IIN>
TA = 25
°
C
MOUNTED ON PCB
VIN
Vref
VOSC
+
+
VFB
IIN
VIN
I
IN
WAVEFORM WHEN THERMAL PROTECTION IS
OPERATING
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