SGS Thomson Microelectronics L4978D, L4978 Datasheet
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
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