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L4978
2A STEP DOWN SWITCHING REGULATOR
1

Features
UP TO 2A STEP DOWN CONVERTER OPERATING INPUT VOLTAGE FROM 8V TO 55V PRECISE 3.3V (1%) INTERNAL REFERENCE VOLTAGE OUTPUT VOLTAGE ADJUSTABLE FROM 3.3V TO 50V SWITCHING FREQUENCY ADJUSTABLE UP TO 300KHz VOLTAGE FEEDFORWARD ZERO LOAD CURRENT OPERATION INTERNAL CURRENT LIMITING (PULSEBYPULSE AND HICCUP MODE) INHIBIT FOR ZERO CURRENT CONSUMPTION PROTECTION AGAINST FEEDBACK DISCONNECTION THERMAL SHUTDOWN SOFT START FUNCTION
Figure 1. Packages
DIP-8
SO16W
Table 1. Order Codes
Part Number L4978 L4978D L4978D013TR Package DIP-8 SO16 SO16 in Tape & Reel

efficency and high switching speed. A switching frequency up to 300KHz is achievable (the maximum power dissipation of the packages must be observed). A wide input voltage range between 8V to 55V and output voltages regulated from 3.3V to 50V cover the majority of today's applications. Features of this new generations of DCDC converter include pulse-by-pulse current limit, hiccup mode for short circuit protection, voltage feedforward regulation, soft-start, protection against feedback loop disconnection, inhibit for zero current consumption and thermal shutdown. The device is available in plastic dual in line, DIP8 for standard assembly, and SO16W for SMD assembly.

2
DESCRIPTION
The L4978 is a step down monolithic power switching regulator delivering 2A at a voltage between 3.3V and 50V (selected by a simple external divider). Realized in BCD mixed technology, the device uses an internal power D-MOS transistor (with a typical Rdson of 0.25) to obtain very high Figure 2. Typical Application Circuit
Vi=8V to 55V 5 R1 20K 3 C1 220F 63V C7 220nF C2 2.7nF 2 7
8
L4978
4 1 6 L1 126H (77120) D1 ST PS3L60U C8 330F VO=3.3V/2A
C5 100nF
R2 9.1K C4 22nF
C6 100nF
D98IN837A
May 2005
Rev. 9 1/13
L4978
Table 2. Block Diagram
VCC 5 THERMAL SHUTDOWN VOLTAGES MONITOR CBOOT CHARGE SS_INH 2 INHIBIT SOFTSTART 3.3V COMP FB 7 8 E/A INTERNAL REFERENCE INTERNAL SUPPLY 5.1V 6 PWM BOOT
R S
3.3V
Q DRIVE
OSCILLATOR 1 GND
CBOOT CHARGE AT LIGHT LOADS
3 OSC
4 OUT
D97IN594
Figure 3. Pins Connection (Top view)
N.C. GND GND SS_INH OSC OUT 1 2 3 4
D97IN595
1 2 3 4 5 6 7 8
D97IN596
16 15 14 13 12 11 10 9
N.C. N.C. FB COMP BOOT VCC N.C. N.C.
8 7 6 5
FB COMP BOOT VCC
SS_INH OSC OUT OUT N.C. N.C.
DIP-8
SO16W
Table 3. Pin Description
N 1 2 Pin 2 3 Name GND SS_INH Ground A logic signal (active low) disables the device (sleep mode operation). A capacitor connected between this pin and ground determines the soft start time. When this pin is grounded disables the device (driven by open collector/drain). An external resistor connected between the unregulated input voltage and this pin and a capacitor connected from this pin to ground fix the switching frequency. (Line feed forward is automatically obtained) Function
3
4
OSC
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L4978
Table 3. Pin Description (continued)
N 4 5 6 7 8 Pin 5, 6 11 12 13 14 Name OUT VCC BOOT COMP FB Stepdown regulator output Unregulated DC input voltage A capacitor connected between this pin and OUT allows to drive the internal DMOS Transistors E/A output to be used for frequency compensation Stepdown feedback input. Connecting directly to this pin results in an output voltage of 3.3V. An external resistive divider is required for higher output voltages. Function
(*) Pins 1, 7, 8, 9, 10, 15 and 16 are not internally, electrically connected to the die.
Table 4. Thermal Data
Symbol Rth(j-amb)
(*) Package mounted on board.
Parameter Thermal Resistance Junction to ambient Max.
Minidip 90 (*)
SO16 110 (*)
Unit C/W
Table 5. Absolute Maximum Ratings
Symbol Parameter Minidip V5 V4 I4 V6-V5 V6 V7 V2 V8 Ptot S016 V11 V5,V6 I5,I6 V12-V11 V12 V13 V3 V14 Bootstrap voltage Analogs input voltage (VCC= 24V) Analogs input voltage (VCC= 24V) (VCC= 20V) Power dissipation a Tamb 60C DIP-8 SO16 Junction and storage temperature 1 0.8 -40 to 150 W W C Input voltage Output DC voltage Output peak voltage at t = 0.1ms f=200KHz Maximum output current 14 70 12 13 6 -0.3 58 -1 -5 int. limit. V V V V V V V V V Value Unit
Tj,Tstg
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L4978
Table 6. Electrical Characteristcs (Tj = 25C, Cosc = 2.7nF, Rosc = 20k, VCC = 24V, unless otherwise specified). "" Specification Referred to Tj from 0 to 125C
Symbol VI Vo Parameter Operating input voltage range Output voltage Test Condition Vo = 3.3 to 50V; Io = 2A Io = 0.5A
Min. 8 3.33 3.292
Typ.
Max. 55
Unit V V V V V V A % KHz dB
DYNAMIC CHARACTERISTIC 3.36 3.36 3.36 0.58

3.39 3.427 3.5 0.733 1.173 3.5 110
Io = 0.2 to 2A
Vcc = 8 to 55V Vd Dropout voltage Maximum limiting current Efficiency fs SVRR Switching frequency Supply voltage ripple rejection Switching Frequency Stability vs. Vcc Temp. stability of switching frequency SOFT START Soft start charge current Soft start discharge current INHIBIT VLL IsLL Iqop Iq Iqst-by Low level voltage Isource Low level Total operating quiescent current Quiescent current Total stand-by quiescent current Duty Cycle = 0; VFB= 3.8V Vinh < 0.9V Vcc = 55V; Vinh<0.9V

3.22
Vcc = 10V; Io = 2A Vcc = 8 to 55V Vo = 3.3V; Io= 2A
Il
2.5 90 60
3 87 100
Vi = Vcc+2VRMS; Vo= Vref; Io = 2.5A; f ripple= 100Hz Vcc = 8 to 55V Tj = 0 to 125C
3 4
6
% %
30 6
40 10
50 14 0.9
A A V A mA mA A A V mV mV/C V
5 4 2.5 100 150 3.33 3.36 5 0.4 10.3
15 6 3.5 200 300 3.39 10
DC CHARACTERISTICS
ERROR AMPLIFIER VFB RL Voltage Feedback Input Line regulation Ref. voltage stability vs temperature VoH VoL Io source Io sink Ib SVRR E/A High level output voltage Low level output voltage Source output current Sink output current Source bias current Supply voltage ripple rejection DC open loop gain gm Transconductance Vcomp = VFB; Vcc = 8 to 55V RL = Icomp = -0.1 to 0.1mA Vcomp = 6V 60 50 VFB = 2.5V VFB = 3.8V Vcomp= 6V; VFB= 2.5V Vcomp = 6V; VFB= 3.8V 180 200 220 300 2 80 57 2.5 3 Vcc = 8 to 55V
0.65
V A A A dB dB mS
4/13
L4978
Table 6. Electrical Characteristcs (Tj = 25C, Cosc = 2.7nF, Rosc = 20k, VCC = 24V, unless otherwise specified). "" Specification Referred to Tj from 0 to 125C
Symbol Parameter Ramp Valley Ramp peak Maximum duty cycle Maximum Frequency Duty Cycle = 0% Rosc = 13k, Cosc = 820pF Vcc = 8V Vcc = 55V Test Condition Min. 0.78 2 9 95 Typ. 0.85 2.15 9.6 97 300 Max. 0.92 2.3 10.2 Unit V V V % kHz OSCILLATOR SECTION
Figure 4. Test and evaluation board circuit.
Vi=8V to 55V 5 R1 20K 3 C1 220F 63V C7 220nF C2 2.7nF 2 7 8
L4978
4 1 6 L1 126H (77120) C8 330F
VO=3.3V/2A
R3
C5 100nF
R2 9.1K C4 22nF
C6 100nF D1 STPS3L60U
R4
D98IN834B
C1=220F/63V EKE C2=2.7nF C5=100nF C6=100nF C7=220nF/63V C8=330F/35V CG Sanyo L1=126H KoolMu 77120 - 55 Turns - 0.5mm R1=20K R2=9.1K D1=STPS3L60U
L4978
VO(V) 3.3 5.1 12 15 18 24 R3(K) 0 2.7 12 16 20 30 4.7 4.7 4.7 4.7 4.7 R4(K)
Figure 5. PCB and component layout of the figure 4.
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L4978
Figure 6. Quiescent drain current vs. input voltage.
Iq (mA)
200KHz R1=22K C2=1.2nF 100KHz R1=20K C2=2.7nF
D97IN724
Figure 9. Line Regulation .
VO (V) 3.377
Tj=125C
D97IN733
5
3.376 3.375
Tj=25C
4
3.374 3
0Hz
3.373 3.372
2
Tamb=25C 0% DC
3.371 3.370
1 0 5 10 15 20 25 30 35 40 45 50 Vcc(V)
0
5 10 15 20 25 30 35 40 45 50 VCC(V)
Figure 7. Quiescent current vs. junction temperature
Iq (mA) 5
D97IN731
Figure 10. Load regulation .
VO (V) 3.378 3.376 3.374 3.372 3.370 3.368
Tj=125C Tj=25C
D98IN835
VCC=35V
200KHz R1=22K C2=1.2nF 100KHz R1=20K C2=2.7nF 0Hz
4
3
VCC=35V 0% DC
3.366 3.364 3.362
2
1 -50 -30 -10 10 30 50 70 90 110 Tj(C)
3.360 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 IO(A)
Figure 8. Stand by drain current vs. input voltage.
Ibias (A) 150 140 130 120 110 100 90 80 70 60 0 5 10 15 20 25 30 35 40 45 50 VCC(V)
Tj=125C Vss=GND Tj=25C
D97IN732
Figure 11. Switching frquency vs. R1 and C2 .
fsw (KHz) 500
0.8
D97IN784
Tamb=25C
200 100 50 20
2nF 1.2 nF
2.2 nF
3.3n
F 4.7n F
5.6n
F
10 5 0 20 40 60 80 R1(K)
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L4978
Figure 12. Switching Frequency vs. input voltage..
fsw (KHz) 107.5 105.0 102.5 100.0
88
D97IN735
Figure 15. Efficiency vs output voltage. .
[%] 98 96 94 92
fsw=100kHz
fsw=200kHz
Tj=25C
90
97.5 95.0 92.5 90.0 0 5 10 15 20 25 30 35 40 45 50 VCC(V)
86 84 82 80
Vcc=35V Io=2A
0
5
10
15 Vo [V]
20
25
30
Figure 13. Switching frequency vs. junction temperature.
fsw (KHz)
D97IN785
Figure 16. Efficiency vs. output current. .
[%] 95
Vcc=8V Vcc=12V
90
105
85
Vcc=24V
80
100
75
Vcc=48V
70
95
65
fsw=100kHz Vo=5.1V
90 -50 0 50 100 Tj(C)
60 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Io [A] 2 2.2
Figure 14. Dropout voltage between pin 5 and 4.
V (V) 0.7 0.6
Tj=125C
D98IN836
Figure 17. Efficiency vs. output current. .
[%] 95 90 85
Vcc=8V
0.5
Tj
=
25
C
80 75
Vcc=12V
0.4
Vcc=24V
0.3 0.2 0.1 0.0
Tj=-25C
70 65 60
Vo=3.36V fsw=100kHz
Vcc=48V
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 IO(A)
0
0.2 0.4 0.6 0.8
1 1.2 1.4 1.6 1.8 Io [A]
2
2.2
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L4978
Figure 18. Efficiency vs. output current. .
(%) 90 85 80
VCC=48V
Io=1.5A
D97IN740
Figure 21. Power dissipation vs. Vcc. .
Pdiss [mW] 1000
VCC=8V VCC=12V VCC=24V
600
Io=1A Vo=5.1V fsw=100kHz
800
Io=2A
75 70 65 60
fsw=200KHz VO=5.1V
400
Io=0.5A
200
0
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 IO(A)
0
10
20
30 Vcc [V]
40
50
60
Figure 19. Efficiency vs. output current. .
(%) 90 85 80 75
VCC=8V VCC=12V VCC=24V
D97IN741
Figure 22. Device Power dissipation vs. Vo .
Pdiss [mW] 1400
Vcc=35V fsw=100kHz Io=2A
1200
1000
Io=1.5A
800
600
Io=1A
70 65 60
VCC=48V fsw=200KHz VO=3.36V
400
Io=0.5A
200
55 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 IO(A)
0 0 10 20 Vo [V] 30
Figure 20. Efficiency vs. Vcc. .
n [%] 90
Vo=5.1V fsw=100kHz
Figure 23. Pulse by pulse limiting current vs. junction temperature..
Ilim (A) 2.9 2.8 2.7 2.6 2.5
fsw=100KHz VCC=35V
D97IN747
85
Vo=5.1V fsw=200kHz Vo=3.36V fsw=100kHz
80
Vo=3.36V fsw=200kHz
75
Io=2A
2.4
70 0 10 20 30 Vcc [V] 40 50 60
2.3 -50 -25 0 25 50 75 100 125 Tj(C)
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L4978
Figure 24. Load transient. . Figure 27. Soft start capacitor selection Vs inductor and Vccmax.
L (H)
D97IN746
fsw=200KHz
56nF
300
47nF
200
33nF
22nF
100
0 15 20 25 30 35 40 45 50 VCCmax(V)
Figure 25. Line transient. .
VCC (V) 30 20
D97IN786
Figure 28. Open loop frequency and phase of error amplifier .
GAIN (dB) 50
GAIN
D97IN787
Phase
0 10 1
IO = 1A fsw = 100KHz
0 45 90
Phase
VO (mV) 100 0
-50 -100 -150
2
135
-100
1ms/DIV
-200 10 102 103 104 105 106 107 108 f(Hz)
Figure 26. Soft start capacitor selection Vs inductor and Vccmax.
L (H)
fsw=100KHz
D97IN745
680nF 470nF
400
330nF
300
200
220nF
100
100nF
0 15 20 25 30 35 40 45 50 VCCmax(V)
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L4978
3
Package Informations
Figure 29. DIP-8 Mechanical Data & Package Dimensions
mm DIM. MIN. A a1 B b b1 D E e e3 e4 F I L Z 3.18 7.95 2.54 7.62 7.62 6.6 5.08 3.81 1.52 0.125 0.51 1.15 0.356 0.204 1.65 0.55 0.304 10.92 9.75 0.313 0.100 0.300 0.300 0.260 0.200 0.150 0.060 TYP. 3.32 0.020 0.045 0.014 0.008 0.065 0.022 0.012 0.430 0.384 MAX. MIN. TYP. 0.131 MAX. inch
OUTLINE AND MECHANICAL DATA
DIP-8
10/13
L4978
Figure 30. SO16 Wide Mechanical Data & Package Dimensions
mm DIM. MIN. A A1 B C D (1) E e H h L k ddd 10.0 0.25 0.40 2.35 0.10 0.33 0.23 10.10 7.40 1.27 10.65 0.75 1.27 0.394 0.010 0.016 TYP. MAX. 2.65 0.30 0.51 0.32 10.50 7.60 MIN. 0.093 0.004 0.013 0.009 0.398 0.291 0.050 0.419 0.030 0.050 TYP. MAX. 0.104 0.012 0.200 0.013 0.413 0.299 inch
OUTLINE AND MECHANICAL DATA
0 (min.), 8 (max.) 0.10 0.004
(1) "D" dimension does not include mold flash, protusions or gate burrs. Mold flash, protusions or gate burrs shall not exceed 0.15mm per side.
SO16 (Wide)
0016021 C
11/13
L4978
Table 7. Revision History
Date October 2001 May 2005 Revision 8 9 First Issue Modified D1 on the Fig. 4. Description of Changes
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L4978
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners (c) 2005 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America www.st.com
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