2A STEP DOWN SWITCHING REGULATOR
UP TO 2A STEP DOWN CONVERTER
OPERATINGINPUT VOLTAGE FROM 8V TO
55V
PRECISE 3.3V (±1%) INTERNAL REFER-
ENCE VOLTAGE
OUTPUT VOLTAGE ADJUSTABLE FROM
3.3VTO 50V
SWITCHINGFREQUENCY ADJUSTABLE UP
TO 500KHz
VOLTAGEFEEDFORWARD
ZEROLOAD CURRENT OPERATION
INTERNAL CURRENT LIMITING (PULSE-BY-
PULSEAND HICCUPMODE)
INHIBIT FOR ZERO CURRENT CONSUMP-
TION
PROTECTION AGAINST FEEDBACK DIS-
CONNECTION
THERMAL SHUTDOWN
SOFTSTART FUNCTION
DESCRIPTION
The L4978 is a step down monolithic power
switching regulator delivering 2A at a voltage between 3.3V and 50V (selectedby a simple external divider). Realized in BCD mixed technology,
the device uses an internal power D-MOS transistor (with a typicalRdson of 0.25Ω) to obtain very
high efficency and high switchingspeed.
TYPICAL APPLICATIONCIRCUIT
L4978
Minidip SO16W
ORDERING NUMBERS: L4978 (Minidip)
L4978D (SO16)
A switching frequency up to 500KHz 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 themajority of today’s applications.
Features of this new generations of DC-DC 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 consumptionand thermalshutdown.
The device is available in plastic dual in line,
MINIDIP 8 for standard assembly, and SO16W
for SMD assembly.
May 2000
Vi=8V to 55V
C1
220µF
63V
C7
220nF
R1
20K
C2
2.7nF
C5
100nF
5
3
2
L4978
7
R2
9.1K
22nF
1
C4
6
C6
100nF
D98IN837A
8
4
D1
GI
SB560
L1
126µ
H
(77120)
C
330µF
VO=3.3V/2A
8
1/12
L4978
BLOCKDIAGRAM
COMP
FB
2
7
8
3.3V
SS_INH
THERMAL
SHUTDOWN
INHIBIT SOFTSTART
E/A
OSCILLATOR
VOLTAGES
MONITOR
PWM
3.3V
INTERNAL
REFERENCE
R
Q
S
INTERNAL
SUPPLY
5.1V
DRIVE
VCC
5
CBOOT
CHARGE
CBOOT
CHARGE
AT LIGHT
LOADS
6
BOOT
3
OSC GND OUT
1
4
D97IN594
PIN CONNECTIONS
16
15
14
13
12
11
10
9
GND
SS_INH
OSC
OUT
1
2
3
4 VCC
D97IN595
FB8
COMP
7
BOOT
6
5
Minidip
N.C.
GND
SS_INH
OSC
OUT
OUT
N.C.
N.C. N.C.
2
3
4
5
6
7
8
D97IN596
SO16W
PIN FUNCTIONS
DIP SO (*) Name Function
1 2 GND Ground
2 3 SS_INH A logic signal (active low) disables the device (sleep mode operation).
3 4 OSC An external resistor connected between the unregulated input voltage and this pin and
4 5, 6 OUT Stepdown regulator output
511 V
CC
6 12 BOOT A capacitor connected between this pin and OUT allows to drive the internal DMOS
7 13 COMP E/A output to be used for frequency compensation
8 14 FB Stepdown feedback input. Connecting directly to this pin results in an output voltage of
(*) Pins 1, 7, 8, 9, 10, 15 and 16 are not internally, electrically connected to the die.
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).
a capacitor connected from this pin to groundfix the switching frequency. (Line feed
forward is automatically obtained)
Unregulated DC input voltage
Transistors
3.3V. An externalresistive divider is required for higher output voltages.
N.C.1
N.C.
FB
COMP
BOOT
VCC
N.C.
2/12
THERMALDATA
Symbol Parameter Minidip SO16 Unit
R
th(j-amb)
(*) Package mounted on board.
Thermal Resistance Junction to ambient Max. 90 (*) 110 (*) °C/W
ABSOLUTE MAXIMUM RATINGS
L4978
Symbol
Minidip S016
V
11
V5,V
I5,I
V12-V
V
12
V
13
V3 Analogs input voltage (VCC= 24V) 13 V
V
14
V
V
5
V
4
I
4
6-V5
V
6
V
7
V
2
V
8
Input voltage 58 V
Output DC voltage
6
Output peak voltage at t = 0.1µs f=200KHz
Maximum output current int. limit.
6
11
Bootstrap voltage 70 V
Analogs input voltage (VCC= 24V) 12 V
(VCC= 20V) 6
Parameter Value Unit
-1
-5
14 V
-0.3
P
tot
Power dissipation a T
≤ 60°C Minidip 1W
amb
SO16 0.8 W
T
j,Tstg
ELECTRICAL CHARACTERISTICS (Tj=25°C, C
wisespecified.) * SpecificationRefered to T
Junction and storage temperature -40 to 150 °C
= 2.7nF, Rosc = 20kΩ,VCC= 24V, unless other-
osc
from0 to 125°C
j
Symbol Parameter Test Condition Min. Typ. Max. Unit
Dynamic Characteristic
V
V
o
V
d Dropout voltage Vcc = 10V;Io = 2A 0.58 0.733 V
I
l Maximum limiting current Vcc = 8 to 55V
f
s Switching frequency
SVRR Supply voltage ripple rejection V
Operating input voltage range Vo= 3.3 to 50V; I
I
=2A • 855V
o
Output voltage Io= 0.5A 3.33 3.36 3.39 V
I
= 0.2 to 2A 3.292 3.36 3.427 V
o
V
= 8to 55V • 3.22 3.36 3.5 V
cc
•
•
2.5 3 3.5 A
Efficiency V
Voltage stability of switching
frequency
Temp. stability of switching
frequency
o = 3.3V; Io =2A 87 %
•
90 100 110 KHz
i =Vcc+2VRMS;Vo=Vref;
I
= 2.5A; f
o
ripple
= 100Hz
60 dB
Vcc = 8 to 55V 3 6 %
j = 0 to 125°C4%
T
1.173 V
V
V
V
V
Soft Start
Soft start charge current 30 40 50 µA
Soft start discharge current 6 10 14 µA
Inhibit
VLL Low level voltage
I
sLL Isource Low level
•
•
515µA
0.9 V
3/12
L4978
ELECTRICAL CHARACTERISTICS (continued)
Symbol Parameter Test Condition Min. Typ. Max. Unit
DC Characteristics
Iqop Total operating quiescent
current
I
q Quiescent current Duty Cycle = 0; V
I
qst-by
Total stand-by quiescent
current
V
<0.9V 100 200 µA
inh
Vcc = 55V; V
= 3.8V 2.5 3.5 mA
FB
<0.9V 150 300 µA
inh
Error Amplifier
V
FB
R
L Line regulation Vcc = 8 to 55V 5 10 mV
V
oH High level output voltage V
V
oL Low level output voltage V
I
o source Source output current V
I
o sink Sink output current V
I
b Source bias current 2 3 µA
SVRR E/A Supply voltage ripple rejection V
gm Transconductance I
Voltage Feedback Input 3.33 3.36 3.35 V
Ref. voltage stability vs
• 0.4 mV/°C
temperature
= 2.5V 10.3 V
FB
= 3.8V 0.65 V
FB
= 6V; VFB= 2.5V 180 220 µA
comp
= 6V; VFB= 3.8V 200 300 µA
comp
; Vcc = 8 to 55V 60 80 dB
= ∞ 50 57 dB
= -0.1 to 0.1mA
=6V
DC open loop gain R
comp=Vfb
L
comp
V
comp
OscillatorSection
Ramp Valley 0.78 0.85 0.92 V
Ramp peak Vcc= 8V 2 2.15 2.3 V
Vcc = 55V 9 9.6 10.2 V
Maximum duty cycle 95 97 %
Maximum Frequency Duty Cycle = 0%
R
osc
= 13kΩ,C
= 820pF
osc
46mA
2.5 mS
500 kHz
4/12
Figure1. Test and evaluation boardcircuit.
L4978
Vi=8V to 55V
R
1
20K
C1
220µ
C
F
7
220nF
C
2
2.7nF
63V
C
5
100nF
C1=220µF/63V EKE
C2=2.7nF
C5=100nF
C6=100nF
C7=220nF/63V
C8=330µF/35V CG Sanyo
L1=126µH KoolMu 77120 - 55 Turns - 0.5mm
R1=20K
R2=9.1K
D1=GI SB560
5
3
2
C
4
22nF
L4978
7
R
2
9.1K
8
V
=3.3V/2A
O
4
L1
1
6
C
6
100nF
D1
GI
SB560
D98IN834A
126µ
(77120)
C
330µF
H
R
3
8
R
4
L4978
(V) R3(KΩ) R4(KΩ)
V
O
3.3
5.1
12
15
18
24
0
2.7
12
16
20
30
4.7
4.7
4.7
4.7
4.7
Figure2. PCB and component layoutof the figure 1.
5/12
L4978
Figure3. Quiescentdrain current vs. input
voltage.
Iq
(mA)
200KHz
R1=22K
5
C2=1.2nF
4
3
2
Tamb=25°
C
0% DC
1
0 5 10 15 20 25 30 35 40 45 50
100KHz
R1=20K
C2=2.7nF
0Hz
D97IN724
Vcc(V)
Figure5. Stand bydrain current vs. input
voltage
Ibias
(µA)
150
140
Vss=GND
Tj=25°C
130
120
110
100
Tj=125°C
90
80
70
60
0 5 10 15 20 25 30 35 40 45 50 VCC(V)
D97IN732
Figure 4. Quiescentcurrent vs. junction
temperature
200KHz
R
=22K
1
=1.2nF
C
2
0Hz
D97IN731
Iq
(mA)
5
4
100KHz
=20K
R
1
=2.7nF
C
2
3
VCC=35V
2
0% DC
1
-50 -30 -10 10 30 50 70 90 110 Tj(°C)
Figure 6. LineRegulation
VO
(V)
3.377
Tj=125°C
3.376
3.375
3.374
3.373
3.372
3.371
3.370
0 5 10 15 20 25 30 35 40 45 50 VCC(V)
D97IN733
Tj=25°C
Figure7. Loadregulation
VO
(V)
3.378
3.376
3.374
3.372
3.370
3.368
3.366
3.364
3.362
3.360
0 0.2 0.4 0.6 1.0 1.2 1.4 1.6 IO(A)0.8 1.8
6/12
Tj=125°C
Tj=25°C
Figure 8. Switchingfrquency vs. R1 and C2
D98IN835
V
=35V
CC
fsw
(KHz)
500
0.82nF
200
1.2nF
100
50
20
2.2nF
3.3nF
4.7nF
5.6nF
10
5
0 20 40 60 80 R1(KΩ)
D97IN784
Tamb=25°C
L4978
Figure9. SwitchingFrequencyvs. input
voltage.
fsw
(KHz)
107.5
105.0
102.5
100.0
97.5
95.0
92.5
90.0
0 5 10 15 20 25 30 35 40 45 50 VCC(V)
D97IN735
Tj=25°C
Figure11. Dropout voltage between pin 5
and 4.
∆V
(V)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 IO(A)1.6 1.8
D98IN836
Tj=125°C
Tj=25°C
Tj=-25°C
Figure 10. Switching frequencyvs. junction
temperature.
fsw
(KHz)
105
100
95
90
-50 0 50 100 Tj(°C)
D97IN785
Figure 12. Efficiency vs output voltage.
η
[%]
98
Vo [V]
fsw=100kHz
fsw=200kHz
Vcc=35V
Io=2A
96
94
92
90
88
86
84
82
80
0 5 10 15 20 25 30
Figure13. Efficiencyvs. output current.
η
[%]
95
Vcc=12V
90
85
80
75
70
65
60
Vcc=12V
Vcc=24V
Vcc=48V
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
Vcc=8V
Vcc=8V
Io [A]
fsw=100kHz
Vo=5.1V
Figure 14. Efficiencyvs.output current.
η
[%]
95
Vcc=8V
90
85
Vcc=12V
80
Vcc=24V
75
70
Io [A]
Vo=3.36V
fsw=100kHz
65
Vcc=48V
60
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
7/12
L4978
Figure15. Efficiencyvs. output current.
η
(%)
90
85
80
=12V
V
CC
V
CC
V
=24V
CC
VCC=8V
=48V
75
70
65
60
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 IO(A)
D97IN740
fsw=200KHz
VO=5.1V
Figure17. Efficiencyvs. Vcc.
n [%]
90
Vo=5.1Vfsw=100kHz
85
80
Vo=5.1V fsw=200kHz
Vo=3.36V fsw=100kHz
Vo=3.36Vfsw=200kHz
Figure 16. Efficiencyvs.output current.
η
(%)
90
85
80
VCC=8V
=12V
V
CC
V
=24V
CC
75
70
V
=48V
CC
65
60
55
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 IO(A)
D97IN741
fsw=200KHz
VO=3.36V
Figure 18. Powerdissipationvs.Vcc.
Pdiss[mW]
1000
Vo=5.1V
fsw=100kHz
800
600
Io=1.5A
400
Io=2A
Io=1A
75
70
Io=2A
0 102030405060
Vcc [V]
Figure19. DevicePowerdissipationvs.Vo
Pdiss [mW]
1400
Vcc=35V
fsw=100kHz
1200
1000
800
600
400
200
0
0 102030
Io=2A
Io=1.5A
Io=1A
Io=0.5A
Vo[V]
200
0
0 102030405060
Vcc[V]
Io=0.5A
Figure 20. Pulsebypulselimitingcurrentvs.
junction temperature.
Ilim
(A)
2.9
fsw=100KHz
VCC=35V
2.8
2.7
2.6
2.5
2.4
2.3
-50 -25 0 25 50 75 100 125 Tj(°C)
D97IN747
8/12
L4978
Figure21. Load transient.
Figure23. Soft start capacitorselection Vs in-
ductor and Vccmax.
L
(µH)
400
300
200
100
680nF
fsw=100KHz
D97IN745
470nF
330nF
220nF
100nF
Figure 22. Line transient.
1A
D97IN786
V
O
(mV)
100
0
-100
V
1
2
(V)
30
20
10
CC
1ms/DIV
IO=
fsw= 100KHz
Figure 24. Softstartcapacitorselectionvs.In-
ductorandVccmax.
L
(µH)
300
200
100
fsw=200KHz
D97IN746
56nF
47nF
33nF
22nF
0
15 20 25 30 35 40 45 50 V
CCmax
(V)
Figure25. Openloopfrequencyandphaseof er-
roramplifier
GAIN
(dB)
50
0
-50
-100
-150
-200
10 10
3
2
10
5
4
10
10
GAIN
Phase
6
10
7
D97IN787
8
10
Phase
0
45
90
135
f(Hz)10
0
15 20 25 30 35 40 45 50 VCCmax(V)
9/12
L4978
DIM.
MIN. TYP. MAX. MIN. TYP. MAX.
A 3.32 0.131
a1 0.51 0.020
B 1.15 1.65 0.045 0.065
b 0.356 0.55 0.014 0.022
b1 0.204 0.304 0.008 0.012
D 10.92 0.430
E 7.95 9.75 0.313 0.384
e 2.54 0.100
e3 7.62 0.300
e4 7.62 0.300
F 6.6 0.260
I 5.08 0.200
L 3.18 3.81 0.125 0.150
Z 1.52 0.060
mm inch
OUTLINE AND
MECHANICAL DATA
Minidip
10/12
L4978
DIM.
MIN. TYP. MAX. MIN. TYP. MAX.
A 2.35 2.65 0.093 0.104
A1 0.1 0.3 0.004 0.012
B 0.33 0.51 0.013 0.020
C 0.23 0.32 0.009
D 10.1 10.5 0.398 0.413
E 7.4 7.6 0.291 0.299
e 1.27 0.050
H 10 10.65 0.394 0.419
h 0.25 0.75 0.010 0.030
L 0.4 1.27 0.016 0.050
K0°(min.)8° (max.)
mm inch
0.013
OUTLINE AND
MECHANICAL DATA
SO16 Wide
L
hx
45
A
B
e
K
A1
C
H
D
16
9
E
1
8
11/12
L4978
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