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SC2620 датащи(PDF) 9 Page - Semtech Corporation

номер детали SC2620
подробное описание детали  Dual 2A, 30V Step-down Regulator with Programmable Frequency up to 1.4MHz
PDF  26 Pages
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производитель  SEMTECH [Semtech Corporation]
домашняя страница  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2620 датащи(HTML) 9 Page - Semtech Corporation

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 2006 Semtech Corp.
www.semtech.com
SC2620
POWER MANAGEMENT
Applications Information
Setting the Output Voltage
The regulator output voltage is set with an external resistive
divider (Figure 4) with its center tap tied to the FB pin.
)
1
V
(
R
R
OUT
2
1
−
=
(1)
The percentage error due the input bias current of the
error amplifier is
V
1
)
R
R
(
100
nA
15
V
V
2
1
OUT
OUT

⋅
⋅
−
=
∆
.
Example: Determine the output voltage error of a
V
5
V
OUT =
converter with
Ω
=
k
1
.
51
R
2
.
From (1),
Ω
=
−
⋅
Ω
=
k
205
)
1
5
(
k
1
.
51
R
1
%
061
.
0
V
1
)
k
205
k
1
.
51
(
100
nA
15
V
V
OUT
OUT
−
=

⋅
⋅
−
=
∆
.
This error is at least an order of magnitude lower than the
ratio tolerance resulting from the use of 1% resistors in the
divider string.
Setting the Channel Frequency
The switching frequency of the master oscillator is set with
an external resistor from the ROSC pin to ground. Channel
frequency is one-half of that of the master oscillator. A
graph of channel frequency against R
OSC is shown in the
“Typical Performance Characteristics”. Channel frequency
is programmable up to 1.4MHz.
Channel switching frequency is limited by the minimum
controllable on time at low duty cycles. For V
IN > 20V, setting
the switching frequency below 500kHz makes converter
output short circuit operation more robust. These will be
described in more details later.
Minimum On Time Consideration
The operating duty cycle of a non-synchronous step-down
switching regulator in continuous-conduction mode (CCM)
is given by
CESAT
D
IN
D
OUT
V
V
V
V
V
D
−
+
+
=
(2)
where V
CESAT is the switch saturation voltage and VD is
voltage drop across the rectifying diode.
Duty cycle decreases with increasing
OUT
IN
V
V
ratio. In peak
current-mode control, the PWM modulating ramp is the
sensed current ramp of the power switch. This current
ramp is absent unless the switch is turned on. The
intersection of this ramp with the output of the voltage
feedback error amplifier determines the switch pulse width.
The propagation delay time required to immediately turn
off the switch after it is turned on is the minimum
controllable switch on time (T
ON(MIN)). Closed-loop
measurement of the SC2620 with low
IN
OUT
V
V
ratios shows
VOUT
R2
R1
15nA
FB
SC2620
Figure 4. V
OUT is set with a Resistive Divider
Minimum On Time
vs Ambient Temperature
80
90
100
110
120
130
-50
-25
0
25
50
75
100
Temperature (°C)
Figure 5. Variation of Minimum On Time with
Ambient Temperature.



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