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A8499 High Voltage Step-Down Regulator
Features and Benefits
8 to 50 V input range Integrated DMOS switch Adjustable fixed off-time Highly efficient Adjustable 1.2 to 24 V output
Description
The A8499 is a step down regulator that will handle a wide input operating voltage range. The A8499 is supplied in a low-profile 8-lead SOIC with exposed pad (package LJ). Applications include: Printer power supplies Consumer equipment power supplies
Package: 8-Lead SOIC with exposed thermal pad (suffix LJ)
Approximate Scale 1:1
Typical Application
+42 V
Efficiency vs. Output Current
CBOOT 0.01 F C3 100 F 50 V BOOT VIN C3 0.22 F
ENB
LX
A8499
TSET
RTSET
L1 47 H
VOUT 3.3 V / 1.2 A
VBIAS R1 17.8 k
121 k GND FB D1
ESR R2 10.2 k COUT 220 F 10 V
90.0 88.0 86.0 84.0 82.0 80.0 78.0 76.0 74.0 72.0 70.0 0 200 400 600 800 1000
Efficiency %
VOUT (V) 3.3 5 1200 1400
IOUT (mA)
Circuit for 42 V step down to 3.3 V at 1.2 A. Efficiency data from circuit shown in left panel.Data is for reference only.
A8499-DS, Rev. 3
A8499
High Voltage Step-Down Regulator
Functional Block Diagram
BOOT
Boot Charge
VIN
+
VIN
VOUT LX D1 L1 ESR COUT
C
ENB
Switch Disable
Switch PWM
TSET
+
I_Peak
-
I_Demand
+
COMP GND
UVLO TSD Soft Start Ramp Generation 1.2 V
Absolute Maximum Ratings
Supply Voltage, VIN ......................................................................... 50 V VBIAS Input Voltage, VBIAS .................................................-0.3 to 7 V Switch Voltage, VLX......................................................................... -1 V ENB Input Voltage, VENB ......................................................-0.3 to 7 V Junction Temperature, TJ(max) ....................................................... 150C Storage Temperature, TS ............................................... -55C to 150C Operating Ambient Temperature, TA............................... -20C to 85C
Ordering Information Use the following complete part numbers when ordering:
Part Numbera A8499SLJTR-T A8499SLJ-T
aLeadframe bContact Allegro
Packingb 13 in. reel, 3000 pieces/reel 98 pieces/tube
plating 100% matte tin. for additional packing options.
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
-
Clamp
-
FB
Bias Supply VBIAS
VBIAS is connected to VOUT when VOUT target is between 3.3 and 5 V
Package Thermal Characteristics*
Package LJ RJA (C/W) 35 PCB 4-layer
* Additional information is available on the Allegro Web site
Description LJ package, SOIC surface mount with exposed thermal pad
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A8499
High Voltage Step-Down Regulator
ELECTRICAL CHARACTERISTICS1,2 at TA = 25C, VIN = 8 to 50 V (unless noted otherwise)
Characteristics Symbol Test Conditions VENB = LOW, IOUT = 0 mA, VIN = 42 V VBIAS = VOUT (see note3) VIN Quiescent Current IVIN(Q) VENB = LOW, IOUT = 0 mA, VIN = 42 V VBIAS < 3 V VENB = HIGH VBIAS Input Current Buck Switch On Resistance Fixed Off-Time Proportion Feedback Voltage Output Voltage Regulation Feedback Input Bias Current Soft Start Time Buck Switch Current Limit ENB Open Circuit Voltage ENB Input Voltage Threshold ENB Input Current VIN Undervoltage Threshold VIN Undervoltage Hysteresis Thermal Shutdown Temperature Thermal Shutdown Hysteresis IFB tss ICL VOC VENB(0) IENB(0) VUVLO VUVLOHYS TJTSD TJ VFB > 0.5 V VFB < 0.5 V Output disabled LOW level input (Logic 0), output enabled VENB = 0 V VIN rising VIN falling Temperature increasing Recovery = TJTSD - TJ VFB IOUT = 0 mA to 2 A IBIAS RDS(on) VBIAS = VOUT TA = 25C, IOUT = 2 A TA = 125C, IOUT = 2 A Based on calculated value Min. - - - - - - -15 1.176 -3 -400 5 2.2 0.5 2.0 - -10 - 0.7 - - Typ. 0.90 4.4 - 3.5 700 - - 1.200 - -100 10 - - - - - 6.9 - 165 15 Max. 1.35 6.35 100 5 800 1.6 15 1.224 3 100 15 3 1.2 7 1.0 -1 7.1 1.1 - - Units mA mA A mA m % V % nA ms A A V V A V V C C
1. Negative current is defined as coming out of (sourcing) the specified device pin. 2. Specifications over the junction temperature range of 0C to 125C are assured by design and characterization. 3. VBIAS is connected to VOUT node when VOUT target level is between 3.3 and 5 V.
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
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A8499
High Voltage Step-Down Regulator
Functional Description
The A8499 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 continuous 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 determines the fixed off-time (see graph in the toff section). VOUT. The output voltage is adjustable from 1.2 to 24 V, based on the combination of the value of the external resistor divider and the internal 1.2 V 3% reference. The voltage can be calculated with the following formula: VOUT = VFB x (1 + R1/R2) (1) Light Load Regulation. To maintain voltage regulation during light load conditions, the switching regulator enters a cycle-skipping 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 output 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 2.2 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 start up the circuitry is run off of the VIN supply. VBIAS should be connected 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 A8499 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 opperation. 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 (s) is: R (2) tOFF = TSET 10 , 1.2 10 where RTSET (k) is the value of the resistor. Results are shown in the following graph:
Resistance vs. Off-Time
17 15 13 11 9 7 5 3 1 12 36 60 84 108 132 156 180
tOFF (s)
RTSET (k)
tON . From the volt-second balance of the inductor, the turn-on time, tON , can be calculated approximately by the equation:
tON =
where
(VOUT + Vf + IOUT RL) tOFF VIN - IOUT RDS(on) - IOUT RL - VOUT
(3)
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. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
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A8499
High Voltage Step-Down Regulator
The switching frequency is calculated as follows:
to the minimum on-time of the switcher. (4) The extension of the off-time is based on the value of the TSET multiplier and the FB voltage, as shown in the following table:
VFB (V) < 0.25 < 0.50 < 0.75 > 0.75 TSET Multiplier 8 x tOFF 4 x tOFF 2 x tOFF tOFF
1 fSW = tON + tOFF
Shorted Load. If the voltage on the FB pin falls below 0.5 V, the regulator will invoke a 0.8 A typical overcurrent limit to handle shorted load condition at the regulator output. For low output voltages at power up and in the case of a shorted output, the offtime is extended to prevent loss of control of the current limit due
Component Selection
L1. The inductor must be rated to handle the total load current. The value should be chosen to keep the ripple current to a reasonable value. The ripple current, IRIPPLE, can be calculated by: IRIPPLE = VL(OFF) x tOFF / L VL(OFF) = VOUT + Vf + IL(AVG) x RL Example: Given VOUT = 5 V, Vf = 0.55 V, VIN = 42 V, ILOAD = 0.5 A, power inductor with L = 180 H and RL = 0.5 Rdc at 55C, tOFF = 7 s, and RDS(on) = 1 . Substituting into equation 6: VL(OFF) = 5 V + 0.55 V+ 0.5 A x 0.5 = 5.8 V Substituting into equation 5: IRIPPLE = 5.8 V x 7 s / 180 H = 225 mA The switching frequency, fSW, can then be estimated by: fSW = 1 / ( tON + tOFF ) tON = IRIPPLE x L / VL(ON) VL(ON) = VIN - IL(AVG) x RDS(on) - IL(AVG) x RL- VOUT Substituting into equation 9: VL(ON) = 42 V - 0.5 A x 1 - 0.5 A x 0.5 - 5 V = 36 V Substituting into equation 8: tON = 225 mA x 180 H / 36 V = 1.12 s Substituting into equation 7: fSW = 1 / (7 s +1.12 s) = 123 kHz (7) (8) (9) (5) (6) Higher inductor values can be chosen to lower the ripple current. This may be an option if it is required to increase the total maximum current available above that drawn from the switching regulator. The maximum total current available, ILOAD(MAX) , is: ILOAD(MAX) = ICL(MIN) x tOFF / L (5)
where ICL(MIN) is 2.2 A, from the Electrical Chracteristics table. D1. The Schottky catch diode should be rated to handle 1.2 times the maximum load current. The voltage rating should be higher than the maximum input voltage expected during all operating conditions. The duty cycle for high input voltages can be very close to 100%. COUT. The main consideration in selecting an output capacitor is voltage ripple on the output. For electrolytic output capacitors, a low-ESR type is recommended. The peak-to-peak output voltage ripple is simply IRIPPLE x ESR. Note that increasing the inductor value can decrease the ripple current. The ESR should be in the range from 50 to 500 m. RTSET Selection. Correct selection of RTSET values will ensure that minimum on time of the switcher is not violated and prevent the switcher from cycle skipping. For a given VIN to VOUT ratio, the RTSET value must be greater than or equal to the value defined by the curve in the chart RTSET Value versus VIN/VOUT , on the next page. Note. The curve represents the minimum RTSET value. When calculating RTSET , be sure to use VIN(max) / VOUT(min). Resistor tolerance should also be considered, so that under no operating conditions the resistance on the TSET pin is allowed to go below the minimum value. 5
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
A8499
13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5
High Voltage Step-Down Regulator
RTSET Value versus VIN /VOUT
Violation of Minimum On-Time
ET
VIN / VOUT
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0
25.0 27.5
um im in M
e lu Va
R of
TS
Safe Operating Area
47.5
50.0
52.5
55.0
60.0
67.5
32.5
45.0
35.0
30.0
17.5
15.0
42.5
40.0
37.5
22.5
20.0
BOOT ENB TSET RTSET 30.1 k GND
Pin-out Diagram
BOOT ENB TSET GND 1 2 3 4 Pad 8 7 6 5 VIN LX VBIAS FB
Terminal List Table
Pin Name BOOT ENB TSET GND NC NC FB VBIAS LX VIN Pad Pin Description Gate drive boost node On/off control logic input Off-time setting Ground No connect No connect Feedback for adjustable regulator Bias supply input Buck switching node Supply input Exposed pad for thermal dissipation Pin Number 1 2 3 4 N/A N/A 5 6 7 8 Pad
10.0
12.5
65.0
62.5
70.0
57.5
RTSET (k )
Typical Application Circuit
+12 V C2 0.1 F CBOOT 0.01 F VIN L1 47 H VOUT 5.0 V / 1.8 A C1 22 F/ 25 V
A 8499
LX VBIAS FB R2 3.16 k R1 10 k
COUT 330 F/ 6.3 V (Aluminum)
D1 B340
12 V step down to 5.0 V at 1.8 A
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
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A8499
High Voltage Step-Down Regulator
Package LJ 8-Pin SOIC
6.20 .244 5.80 .228 0.25 [.010] M B M 8 5.00 .197 4.80 .189 A B 8 0 0.25 .010 0.17 .007
B
4.00 .157 3.80 .150 2.41 .095 NOM
A
1.27 .050 0.40 .016
3.30 .130 NOM 8X 0.10 [.004] C 8X 0.51 .020 0.31 .012
1
2 0.25 .010 SEATING PLANE 1.75 .069 1.35 .053 C SEATING PLANE GAUGE PLANE
0.25 [.010] M C A B 1.27 .050
0.25 .010 0.10 .004
0.65 .026 MAX
1.27 .050 NOM
1.75 .069 NOM
2X 0.20 .008 MIN
2.41 .095 NOM C
5.60 .220 NOM
All dimensions reference, not for tooling use (reference JEDEC MS-012 AA) Dimensions in millimeters U.S. Customary dimensions (in.) in brackets, for reference only Dimensions exclusive of mold flash, gate burrs, and dambar protrusions Exact case and lead configuration at supplier discretion within limits shown A Terminal #1 mark area B Exposed thermal pad (bottom surface) C Reference land pattern layout (reference IPC7351 SOIC127P600X175-9AM); adjust as necessary to meet application process requirements and PCB layout tolerances; when mounting on a multilayer PCB, thermal vias at the exposed thermal pad land can improve thermal dissipation (reference EIA/JEDEC Standard JESD51-5)
1 3.30 .130 NOM
2 6X 0.20 .008 MIN
The products described herein are manufactured under one or more patents pending. Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro products are not authorized for use as critical components in life-support appliances, devices, or systems without express written approval. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use; nor for any infringements of patents or other rights of third parties that may result from its use. Copyright (c) 2005, 2006 Allegro MicroSystems, Inc.
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
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