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부품번호 A8697 기능
기능 Wide Input Voltage 4.0 A Step Down Regulator
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A8697 데이터시트, 핀배열, 회로
A8697
Wide Input Voltage 4.0 A Step Down Regulator
Features and Benefits
8 to 25 V input range
Integrated DMOS switch
Adjustable fixed off-time
Adjustable output
Package: 8-Lead SOIC with exposed
thermal pad (suffix LJ)
Description
The A8697 is a constant off-time current mode step-down
regulator with a wide input voltage range. Regulation voltage is
set by external resistors, to output voltages as low as 0.8 V.
The A8697 includes an integrated power DMOS switch to
reduce the total solution footprint. It also features internal
compensation, allowing users to design stable regulators with
minimal design efforts.
The off-time can be set with an external resistor, allowing
flexibility in inductor selection. Additionally, the A8697 has
a logic level enable pin which can shut the device down and
put it into a low quiescent current mode for power sensitive
applications.
The A8697 is supplied in a low-profile 8-lead SOIC with
exposed pad (package LJ). Applications include:
Applications with 8 to 25 V input
Consumer electronics, networking equipment
12 V lighter-powered applications (portable DVD, etc.)
Point of Sale (POS) applications
www.DataSheet4U.com
Approximate Scale 1:1
Typical Application
CBOOT
0.01 μF
VIN
+12 V
+ CIN
47 μF
25 V
BOOT
VIN
RTSET
51.1 k7
ENB
A8697
LX
TSET VBIAS
GND FB
Ratings:
L: CDRH104R-3R8
COUT: EEUFM1V151
CIN: EEVFC1H470P
L
3.8 μH
D1 R1
2 k7
R2
6.14 k7
Circuit for 12 V step down to 1.06 V at 4 A
VOUT
1.06 V/ 4.0 A
+ COUT
150 μF
56 mΩ
Efficiency versus Load Current
90
80 VOUT = 2.5 V
70
VOUT = 1.8 V
60
VOUT = 1.06 V
50
40
30
20
10
0
01234
Load Current (A)
Efficiency curve for circuit at left
A8697-DS, Rev. 2




A8697 pdf, 반도체, 판매, 대치품
A8697
Wide Input Voltage 4.0 A Step Down Regulator
ELECTRICAL CHARACTERISTICS1,2 at TA = 25°C, VIN = 8 to 25 V (unless noted otherwise)
Characteristics
Symbol
Test Conditions
Min.
VIN Quiescent Current
VBIAS Input Current
Buck Switch On Resistance
Fixed Off-Time Proportion
IVIN(Q)
IBIAS
RDS(on)
VENB = LOW, VIN = 12 V, VBIAS = 3.2 V,
VFB = 1.5 V (not switching)
VENB = LOW, VIN = 12 V, VBIAS < 3 V,
VFB = 1.5 V
VENB = HIGH
VBIAS = VOUT
TA = 25°C, IOUT = 3 A
Based on calculated value
–15
Feedback Voltage
VFB
0.784
Output Voltage Regulation
VOUT
IOUT = 0 mA to 3 A
–3
Feedback Input Bias Current
IFB
–400
Soft Start Time
tss
5
Buck Switch Current Limit
ICL
VFB > 0.4 V
VFB < 0.4 V
4.8
ENB Open Circuit Voltage
VOC Output disabled
2.0
ENB Input Voltage Threshold
VENB(0)
LOW level input (Logic 0), output enabled
ENB Input Current
IENB(0)
VENB = 0 V
–10
VIN Undervoltage Threshold
VUVLO
VIN rising
6.6
VIN Undervoltage Hysteresis
VUVLO(hys) VIN falling
0.7
Thermal Shutdown Temperature
TJTSD
Temperature increasing
Thermal Shutdown Hysteresis
TJTSD(hys) Recovery = TJTSD – TJTSD(hys)
1Negative current is defined as coming out of (sourcing) the specified device pin.
2Specifications over the junction temperature range of 0ºC to 125ºC are assured by design and characterization.
Typ.
1.0
4.1
3.8
180
0.8
–100
10
6.2
2.7
6.9
165
15
Max.
100
5
15
0.816
3
100
15
7.2
7
1.0
–1
7.2
1.1
Units
mA
mA
μA
mA
mΩ
%
V
%
nA
ms
A
A
V
V
μA
V
V
°C
°C
Allegro MicroSystems, Inc.
4
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com

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A8697 전자부품, 판매, 대치품
A8697
Wide Input Voltage 4.0 A Step Down Regulator
Functional Description
The A8697 is a fixed off-time, current-mode–controlled buck
switching regulator. The regulator requires an external clamping
diode, inductor, and filter capacitor, and operates in both continu-
ous and discontinuous modes. An internal blanking circuit is used
to filter out transients resulting from the reverse recovery of the
external clamp diode. Typical blanking time is 200 ns.
The value of a resistor between the TSET pin and ground deter-
mines the fixed off-time (see graph in the tOFF section).
VOUT. The output voltage is adjustable from 0.8 to 20 V, based on
the combination of the value of the external resistor divider and
the internal 0.8 V ±2% reference. The voltage can be calculated
with the following formula:
VOUT = VFB × (1 + R1/R2)
(1)
Light Load Regulation. To maintain voltage regulation during
light load conditions, the switching regulator enters a cycle-skip-
ping mode. As the output current decreases, there remains some
energy that is stored during the power switch minimum on-time.
In order to prevent the output voltage from rising, the regulator
skips cycles once it reaches the minimum on-time, effectively
making the off-time larger.
Soft Start. An internal ramp generator and counter allow the out-
put to slowly ramp up. This limits the maximum demand on the
external power supply by controlling the inrush current required
to charge the external capacitor and any dc load at startup.
Internally, the ramp is set to 10 ms nominal rise time. During soft
start, current limit is 3.5 A minimum.
The following conditions are required to trigger a soft start:
• VIN > 6 V
• ENB pin input falling edge
• Reset of a TSD (thermal shut down) event
VBIAS. To improve overall system efficiency, the regulator output,
VOUT, is connected to the VBIAS input to supply the operating
bias current during normal operating conditions. During startup
the circuitry is run off of the VIN supply. VBIAS should be con-
nected to VOUT when the VOUT target level is between 3.3 and
5 V. If the output voltage is less than 3.3 V, then the A8697 can
operate with an internal supply and pay a penalty in efficiency,
as the bias current will come from the high voltage supply, VIN.
VBIAS can also be supplied with an external voltage source. No
power-up sequencing is required for normal operation.
ON/OFF Control. The ENB pin is externally pulled to ground
to enable the device and begin the soft start sequence. When the
ENB is open circuited, the switcher is disabled and the output
decays to 0 V.
Protection. The buck switch will be disabled under one or more
of the following fault conditions:
• VIN < 6 V
• ENB pin = open circuit
• TSD fault
When the device comes out of a TSD fault, it will go into a soft
start to limit inrush current.
tOFF. The value of a resistor between the TSET pin and ground
determines the fixed off-time. The formula to calculate tOFF (μs)
is:
tOFF
=
RSET
1–0.03 VBIAS
10.2 × 109
,
(2)
where RTSET (kΩ) is the value of the resistor. Results are shown
in the following graph:
Off-Time Setting versus Resistor Value
200
180
160
140 VBIAS = 5 V
120
100
80 VBIAS = 3.3 V
60
40
20
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
tOFF (µs)
tON. From the volt-second balance of the inductor, the turn-on
time, ton , can be calculated approximately by the equation:
tON =
(VOUT + Vf + IOUT RL) tOFF
VIN IOUT RDS(on) IOUT RL VOUT
(3)
where
Vf is the voltage drop across the external Schottky diode,
RL is the winding resistance of the inductor, and
RDS(on) is the on-resistance of the switching MOSFET.
Allegro MicroSystems, Inc.
7
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com

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