1/13
L4978
May 2005
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 (PULSE-
BYPULSE AND HICCUP MODE)
■INHIBIT FOR ZERO CURRENT
CONSUMPTION
■PROTECTION AGAINST FEEDBACK
DISCONNECTION
■THERMAL SHUTDOWN
■SOFT START FUNCTION
2 DESCRIPTION
The L4978 is a step down monolithic power
switching regulator delivering 2A at a voltage be-
tween 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 R
dson
of 0.25Ω) to obtain very high
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 be-
tween 8V to 55V and output voltages regulated
from 3.3V to 50V cover the majority of today’s ap-
plications. 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 consumption and thermal shutdown.
The device is available in plastic dual in line, DIP-
8 for standard assembly, and SO16W for SMD as-
sembly.
2A STEP DOWN SWITCHING REGULATOR
Figure 2. Typical Application Circuit
D98IN837A
5
2
8
4
1
L4978
C
1
220µF
63V
C
8
330µF
V
O
=3.3V/2A
Vi=8V to 55V
R
1
20K
C
2
2.7nF
R
2
9.1K
C
4
22nF
3
7L1
126µH
(77120)
6
D1
ST
PS3L60U
C
5
100nF
C
7
220nF
C
6
100nF
Rev. 9
Fi
gure 1.
P
ac
k
ages
Table 1. Order Codes
Part Number Package
L4978 DIP-8
L4978D SO16
L4978D013TR SO16 in Tape & Reel
DIP-8 SO16W
L4978
2/13
Table 2. Block Diagram
Figure 3. Pins Connection (Top view)
Table 3. Pin Description
N° Pin Name Function
1 2 GND Ground
2 3 SS_INH 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).
3 4 OSC 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)
INHIBIT SOFTSTART
VOLTAGES
MONITOR
THERMAL
SHUTDOWN
E/A PWM
3.3V
OSCILLATOR
R
SQ
INTERNAL
REFERENCE
INTERNAL
SUPPLY
3.3V 5.1V
DRIVE
CBOOT
CHARGE
CBOOT
CHARGE
AT LIGHT
LOADS
2
7
8
FB
COMP
SS_INH
314
6
5
BOOT
OSC GND OUT
VCC
D97IN594
GND
SS_INH
OSC
OUT
1
3
2
4 VCC
BOOT
COMP
FB8
7
6
5
D97IN595
N.C.
GND
SS_INH
OSC
OUT
N.C.
OUT
N.C. N.C.
N.C.
BOOT
VCC
COMP
FB
N.C.
N.C.1
3
2
4
5
6
7
8
14
13
12
11
10
9
15
16
D97IN596
DIP-8 SO16W
3/13
L4978
(*) Pins 1, 7, 8, 9, 10, 15 and 16 are not internally, electrically connected to the die.
Table 4. Thermal Data
(*) Package mounted on board.
Table 5. Absolute Maximum Ratings
4 5, 6 OUT Stepdown regulator output
5 11 V
CC
Unregulated DC input voltage
6 12 BOOT A capacitor connected between this pin and OUT allows to drive the internal DMOS
Transistors
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 3.3V. An external resistive divider is required for higher output voltages.
Symbol Parameter Minidip SO16 Unit
R
th(j-amb)
Thermal Resistance Junction to ambient Max. 90 (*) 110 (*) °C/W
Symbol
Parameter Value Unit
Minidip S016
V
5
V
11
Input voltage 58 V
V
4
V
5
,V
6
Output DC voltage
Output peak voltage at t = 0.1ms f=200KHz
-1
-5 V
V
I
4
I
5
,I
6
Maximum output current int. limit.
V
6
-V
5
V
12
-V
11
14 V
V
6
V
12
Bootstrap voltage 70 V
V
7
V
13
Analogs input voltage (V
CC
= 24V) 12 V
V
2
V
3
Analogs input voltage (V
CC
= 24V) 13 V
V
8
V
14
(VCC= 20V) 6
-0.3
V
V
P
tot
Power dissipation a Tamb ≤ 60°C
DIP-8
SO16
1
0.8
W
W
T
j
,T
stg
Junction and storage temperature -40 to 150 °C
Table 3. Pin Description (continued)
N° Pin Name Function
L4978
4/13
Table 6. Electrical Characteristcs
(T
j
= 25°C, C
osc
= 2.7nF, R
osc
= 20kΩ, V
CC
= 24V, unless otherwise specified). “●" Specification Referred
to T
j
from 0 to 125°C
Symbol Parameter Test Condition Min. Typ. Max. Unit
DYNAMIC CHARACTERISTIC
V
I
Operating input voltage range V
o
= 3.3 to 50V; I
o
= 2A ●8 55 V
V
o
Output voltage I
o
= 0.5A 3.33 3.36 3.39 V
I
o
= 0.2 to 2A
V
cc
= 8 to 55V
3.292 3.36 3.427 V
●3.22 3.36 3.5 V
V
d
Dropout voltage V
cc
= 10V; I
o
= 2A 0.58 0.733 V
●1.173 V
I
l
Maximum limiting current V
cc
= 8 to 55V ●2.5 3 3.5 A
Efficiency V
o
= 3.3V; I
o
= 2A 87 %
f
s
Switching frequency ●90 100 110 KHz
SVRR Supply voltage ripple rejection V
i
= V
cc
+2V
RMS
; V
o
= V
ref
;
I
o
= 2.5A; f
ripple
= 100Hz
60 dB
Switching Frequency Stability
vs. V
cc
V
cc
= 8 to 55V 3 6 %
Temp. stability of switching
frequency
T
j
= 0 to 125°C 4 %
SOFT START
Soft start charge current 30 40 50 µA
Soft start discharge current 6 10 14 µA
INHIBIT
V
LL
Low level voltage ●0.9 V
I
sLL
Isource Low level ●5 15 µA
DC CHARACTERISTICS
I
qop
Total operating quiescent
current
4 6 mA
I
q
Quiescent current Duty Cycle = 0; VFB= 3.8V 2.5 3.5 mA
I
qst-by
Total stand-by quiescent
current
V
inh
< 0.9V 100 200 µA
V
cc
= 55V; V
inh
<0.9V 150 300 µA
ERROR AMPLIFIER
V
FB
Voltage Feedback Input 3.33 3.36 3.39 V
R
L
Line regulation V
cc
= 8 to 55V 5 10 mV
Ref. voltage stability vs
temperature
●0.4 mV/°C
V
oH
High level output voltage V
FB
= 2.5V 10.3 V
V
oL
Low level output voltage V
FB
= 3.8V 0.65 V
I
o source
Source output current V
comp
= 6V; V
FB
= 2.5V 180 220 µA
I
o sink
Sink output current V
comp
= 6V; V
FB
= 3.8V 200 300 µA
I
b
Source bias current 2 3 µA
SVRR E/A Supply voltage ripple rejection V
comp
= V
FB
; V
cc
= 8 to 55V 60 80 dB
DC open loop gain R
L
= ∞ 50 57 dB
g
m
Transconductance I
comp
= -0.1 to 0.1mA
V
comp
= 6V
2.5 mS
5/13
L4978
Figure 4. Test and evaluation board circuit.
Figure 5. PCB and component layout of the figure 4.
OSCILLATOR SECTION
Ramp Valley 0.78 0.85 0.92 V
Ramp peak V
cc
= 8V 2 2.15 2.3 V
V
cc
= 55V 9 9.6 10.2 V
Maximum duty cycle 95 97 %
Maximum Frequency Duty Cycle = 0%
R
osc
= 13kΩ, C
osc
= 820pF
300 kHz
Table 6. Electrical Characteristcs
(T
j
= 25°C, C
osc
= 2.7nF, R
osc
= 20kΩ, V
CC
= 24V, unless otherwise specified). “●" Specification Referred
to T
j
from 0 to 125°C
Symbol Parameter Test Condition Min. Typ. Max. Unit
D98IN834B
5
2
8
4
1
L4978
C1
220µF
63V
C8
330µF
VO=3.3V/2A
Vi=8V to 55V
R1
20K
C2
2.7nF
R2
9.1K
C4
22nF
3
7L1
126µH
(77120)
6
D1
STPS3L60U
C5
100nF
C7
220nF
C6
100nF
R3
R4
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=STPS3L60U
VO(V) R3(KΩ) R4(KΩ)
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
L4978
L4978
6/13
Figure 6. Quiescent drain current vs. input
voltage.
Figure 7. Quiescent current vs. junction
temperature
Figure 8. Stand by drain current vs. input
voltage.
Figure 9. Line Regulation
.
Figure 10. Load regulation
.
Figure 11. Switching frquency vs. R1 and C2
.
0 5 10 15 20 25 30 35 40 45 50 Vcc(V)
1
2
3
4
5
Iq
(mA)
200KHz
R1=22K
C2=1.2nF
0Hz
D97IN724
Tamb=25˚C
0% DC
100KHz
R1=20K
C2=2.7nF
-50 -30 -10 10 30 50 70 90 110 Tj(˚C)
1
2
3
4
5
Iq
(mA)
D97IN731
0Hz
200KHz
R1=22K
C2=1.2nF
VCC=35V
0% DC
100KHz
R1=20K
C2=2.7nF
0 5 10 15 20 25 30 35 40 45 50 VCC(V)
60
70
80
90
100
110
120
130
140
150
Ibias
(µA)
D97IN732
Tj=25˚C
Tj=125˚C
Vss=GND
0 5 10 15 20 25 30 35 40 45 50 V
CC
(V)
3.370
3.371
3.372
3.373
3.374
3.375
3.376
3.377
V
O
(V)
D97IN733
Tj=25˚C
Tj=125˚C
0 0.2 0.4 0.6 1.0 1.2 1.4 1.6 I
O
(A)0.8 1.8
3.360
3.362
3.364
3.366
3.368
3.370
3.372
3.374
3.376
3.378
V
O
(V)
Tj=25˚C
Tj=125˚C
V
CC
=35V
D98IN835
0 20 40 60 80 R1(K
Ω
)
5
10
20
50
100
200
500
fsw
(KHz)
D97IN784
0.82nF
1.2nF
2.2nF
3.3nF
4.7nF
5.6nF
Tamb=25˚C
7/13
L4978
Figure 12. Switching Frequency vs. input
voltage..
Figure 13. Switching frequency vs. junction
temperature.
Figure 14. Dropout voltage between pin 5 and
4.
Figure 15. Efficiency vs output voltage.
.
Figure 16. Efficiency vs. output current.
.
Figure 17. Efficiency vs. output current.
.
0 5 10 15 20 25 30 35 40 45 50 V
CC
(V)
90.0
92.5
95.0
97.5
100.0
102.5
105.0
107.5
fsw
(KHz)
D97IN735
Tj=25˚C
-50 0 50 100 T
j(
˚C
)
90
95
100
105
fsw
(KHz)
D97IN785
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 IO(A)1.6 1.8
0.0
0.1
0.2
0.3
0.4
0.5
∆V
(V)
0.6
0.7
Tj=25˚C
Tj=125˚C
D98IN836
Tj=-25˚C
0 5 10 15 20 25 30
80
82
84
86
88
90
92
94
96
98
Vo [V]
[%]
fsw=100kHz
fsw=200kHz
Vcc=35V
Io=2A
Vcc=12V
Vcc=8V
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
60
65
70
75
80
85
90
95
Io [A]
[%]
Vcc=8V
Vcc=12V
Vcc=24V
Vcc=48V
fsw=100kHz
Vo=5.1V
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
60
65
70
75
80
85
90
95
Io
[
A
]
[%]
Vcc=8V
Vcc=12V
Vcc=24V
Vcc=48V
Vo=3.36V
fsw=100kHz
L4978
8/13
Figure 18. Efficiency vs. output current.
.
Figure 19. Efficiency vs. output current.
.
Figure 20. Efficiency vs. Vcc.
.
Figure 21. Power dissipation vs. Vcc.
.
Figure 22. Device Power dissipation vs. Vo
.
Figure 23. Pulse by pulse limiting current vs.
junction temperature..
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 IO(A)
60
65
70
75
80
85
90
η
(%)
D97IN740
VCC=8V
fsw=200KHz
VO=5.1V
VCC=12V
VCC=24V
VCC=48V
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 IO(A)
55
60
65
70
75
80
85
90
η
(%)
D97IN741
VCC=8V
fsw=200KHz
VO=3.36V
VCC=12V
VCC=24V
VCC=48V
Io=2A
0 102030405060
70
75
80
85
90
Vcc [V]
n [%]
Vo=5.1V fsw=100kHz
Vo=5.1V fsw=200kHz
Vo=3.36V fsw=100kHz
Vo=3.36V fsw=200kHz
Io=0.5A
Io=2A
Io=1A
0 102030405060
0
200
400
600
800
1000
Vcc [V]
Pdiss [mW]
Io=1.5A
Vo=5.1V
fsw=100kHz
Io=2A
Io=1A
0102030
0
200
400
600
800
1000
1200
1400
Vo [V]
Pdiss [mW]
Io=0.5A
Io=1.5A
Vcc=35V
fsw=100kHz
-50 -25 0 25 50 75 100 125 T
j(
˚C
)
2.3
2.4
2.5
2.6
2.7
2.8
2.9
Ilim
(A)
D97IN747
fsw=100KHz
V
CC
=35V
9/13
L4978
Figure 24. Load transient.
.
Figure 25. Line transient.
.
Figure 26. Soft start capacitor selection Vs
inductor and V
ccmax
.
Figure 27. Soft start capacitor selection Vs
inductor and V
ccmax
.
Figure 28. Open loop frequency and phase of
error amplifier .
2
1
D97IN786
V
CC
(V)
30
20
10
V
O
(mV)
100
0
-100
1ms/DIV
I
O
= 1A
f
sw
= 100KHz
15 20 25 30 35 40 45 50 V
CCmax
(V)
0
100
200
300
400
L
(µH)
D97IN745
680nF
fsw=100KHz 470nF
330nF
220nF
100nF
15 20 25 30 35 40 45 50 V
CC
max(V)
0
100
200
300
L
(µH)
56nF
fsw=200KHz
D97IN746
47nF
33nF
22nF
10 10
3
10
5
10
7
f
(
Hz
)
10
2
10
4
10
6
10
8
-200
-150
-100
-50
GAIN
(dB)
0
50
Phase
0
45
90
135
D97IN787
GAIN
Phase
L4978
10/13
3 Package Informations
Figure 29. DIP-8 Mechanical Data & Package Dimensions
OUTLINE AND
MECHANICAL DATA
DIM.
mm inch
MIN. TYP. MAX. MIN. TYP. MAX.
A3.32 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
e2.54 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
DIP-8
11/13
L4978
Figure 30. SO16 Wide Mechanical Data & Package Dimensions
OUTLINE AND
MECHANICAL DATA
DIM.
mm inch
MIN. TYP. MAX. MIN. TYP. MAX.
A 2.35 2.65 0.093 0.104
A1 0.10 0.30 0.004 0.012
B 0.33 0.51 0.013 0.200
C 0.23 0.32 0.009 0.013
D (1) 10.10 10.50 0.398 0.413
E 7.40 7.60 0.291 0.299
e 1.27 0.050
H 10.0 10.65 0.394 0.419
h 0.25 0.75 0.010 0.030
L 0.40 1.27 0.016 0.050
k 0˚ (min.), 8˚ (max.)
ddd 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
L4978
12/13
Table 7. Revision History
Date Revision Description of Changes
October 2001 8 First Issue
May 2005 9 Modified D1 on the Fig. 4.
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
© 2005 STMicroelectronics - All rights reserved
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13/13
L4978
