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參數(shù)資料
| 型號： | ISL6723AABZ |
| 廠商： | INTERSIL CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Flexible Single Ended Current Mode PWM Controllers |
| 中文描述： | 1 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | ROHS COMPLIANT, PLASTIC, MS-012AC, SOIC-16 |
| 文件頁數(shù)： | 12/23頁 |
| 文件大?。?/td> | 428K |
| 代理商： | ISL6723AABZ |
12
FN9237.0
October 19, 2005
For flyback topologies, Vn can be solved for in terms of
output voltage, current transducer components, and primary
inductance yielding
T
V
N
CT
L
P
N
S
where R
CS
is the current sense burden resistor, N
CT
is the
current transformer turns ratio, L
P
is the primary inductance,
V
O
is the output voltage, and Ns and Np are the secondary
and primary turns, respectively. If a current transformer is not
used, then N
CT
= 1.
For discussion purposes, only the flyback topology will be
further discussed. A discussion addressing the buck derived
topologies may be found in the datasheet for the ISL6753.
The current sense signal, which represents the primary
current after it has been reflected through the current sense
transformer and passed through the current sense burden
resistor, is:
1
D
–
(
)
V
T
2L
P
where V
CS
is the voltage across the current sense resistor
and I
O
is the output current at current limit.
Since the peak current limit threshold is determined by ISET,
the total current feedback signal plus the external ramp
voltage must sum to this value when the output load is at the
current limit threshold.
where the internal gain and offset voltages of the IC have
been included. Substituting Equations 15 and 17 into
Equation 18 and solving for R
CS
yields
ISET
0.1
–
(
P
L
P
N
S
Adding slope compensation is accomplished in the
ISL6722A, ISL6723A using the SLOPE input. The value of
Ve determined in Equation 16, multiplied 10x, is the voltage
required at the SLOPE input.
where I
SLOPE
is the internal charging current on SLOPE,
nominally 53μA.
Over and Under Voltage Monitor
The OV and UV signals are inputs to a window comparator
used to monitor the input voltage level to the converter. If the
voltage falls outside of the user designated operating range,
a shutdown fault occurs. For OV faults, the supply current,
I
CC
, is reduced to 200μA for ~ 295ms at which time recovery
is attempted. If the fault is cleared, a soft-start cycle begins.
Otherwise another shutdown cycle occurs. A UV condition
also results in a shutdown fault, but the device does not
enter the low power mode and no restart delay occurs when
the fault clears.
A resistor divider between Vin and LGND to each input
determines the operational thresholds. The UV threshold
has a fixed hysteresis of 75mV nominal.
Overcurrent Operation
The overcurrent threshold level is set by the voltage applied
at the ISET pin. Setting the overcurrent level may be
accomplished by using a resistor divider network from VREF
to LGND. The ISET threshold should be set at a level that
corresponds to the desired peak output inductor current plus
the additive effects of slope compensation.
Overcurrent delayed shutdown is enabled once the soft-start
cycle is complete. If an overcurrent condition is detected, the
soft-start charging current source is disabled and the
discharging current source is enabled. The soft-start
capacitor is discharged at a rate of 40μA. At the same time a
50μs retriggerable one-shot timer is activated. It remains
active for 50μs after the overcurrent condition stops. The
soft-start discharge cycle cannot be reset until the one-shot
timer becomes inactive. If the soft-start capacitor discharges
by more then 0.125V to 4.375V, the output is disabled and
the soft-start capacitor is discharged. The output remains
disabled and I
CC
drops to 200μA for approximately 295ms.
A new soft-start cycle is then initiated. The shutdown and
restart behavior of the OC protection is often referred to as
hiccup operation due to its repetitive start-up and shutdown
characteristic.
If the overcurrent condition ceases at least 50μs prior to the
soft-start voltage reaching 4.375V, the soft-start charging
and discharging currents revert to normal operation and the
soft-start voltage is allowed to recover.
Hiccup OC protection may be defeated by setting ISET to a
voltage that exceeds the Error Amplifier current control
voltage, or about 1.5V.
V
e
-----------------------------------------
P
-------
1
π
--
D
0.5
–
+
=
V
(EQ. 16)
V
CS
R
N
CT
-----------
I
O
N
S
N
P
-------
-----------------------------------------------
N
P
N
S
-------
+
=
V
(EQ. 17)
V
e
V
CS
0.8
(
)
0.100
+
+
ISET
=
(EQ. 18)
R
CS
)
N
-------------------------------------
1
π
--
0.6D
0.1
–
+
0.8I
O
S
N
P
-------
+
-----------------------------------------------------------------------------------------------------------------------
=
(EQ. 19)
C
SLOPE
I
T
V
e
10
-----------------------------------
=
(EQ. 20)
ISL6722A, ISL6723A
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