ST L4978 User Manual
L4978
Fi
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 (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 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 R
of 0.25Ω) to obtain very high
dson
gure 1. Packages
DIP-8 SO16W
Table 1. Order Codes
Part Number Package
L4978 DIP-8
L4978D SO16
L4978D013TR 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.
Figure 2. Typical Application Circuit
Vi=8V to 55V
R
1
20K
C
2
2.7nF
C
5
100nF
May 2005
C
1
220µF
63V
C
7
220nF
5
3
2
L4978
7
R
9.1K
22nF
1
2
C
4
6
C
6
100nF
8
4
D1
ST
PS3L60U
D98IN837A
L1
126µH
(77120)
C
330µF
VO=3.3V/2A
8
Rev. 9
1/13
L4978
Table 2. Block Diagram
THERMAL
SHUTDOWN
FB
2
INHIBIT SOFTSTART
7
8
3.3V
E/A
OSCILLATOR
SS_INH
COMP
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
Figure 3. Pins Connection (Top view)
GND
SS_INH
OSC
OUT
1
2
3
4 VCC
D97IN595
7
6
5
DIP-8 SO16W
Table 3. Pin Description
FB8
COMP
BOOT
1
4
N.C.
GND
SS_INH
OSC
OUT
OUT
N.C.
N.C. N.C.
2
3
4
5
6
7
8
D97IN596
16
15
14
13
12
11
10
9
D97IN594
N.C.1
N.C.
FB
COMP
BOOT
VCC
N.C.
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)
2/13
Table 3. Pin Description (continued)
N° Pin Name Function
4 5, 6 OUT Stepdown regulator output
L4978
5 11 V
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
(*) Pins 1, 7, 8, 9, 10, 15 and 16 are not internally, electrically connected to the die.
Unregulated DC input voltage
CC
Transistors
of 3.3V. An external resistive divider is required for higher output voltages.
Table 4. Thermal Data
Symbol Parameter Minidip SO16 Unit
R
th(j-amb)
(*) Package mounted on board.
Thermal Resistance Junction to ambient Max. 90 (*) 110 (*) °C/W
Table 5. Absolute Maximum Ratings
Symbol
Parameter Value Unit
Minidip S016
V
V
V
I
6-V5
V
V
V
V
5
4
4
6
7
2
8
V
P
tot
T
j,Tstg
V
V
5,V6
I
12-V11
V
V
V
V
Input voltage 58 V
11
Output DC voltage
Output peak voltage at t = 0.1ms f=200KHz
Maximum output current int. limit.
5,I6
14 V
Bootstrap voltage 70 V
12
Analogs input voltage (VCC= 24V) 12 V
13
Analogs input voltage (VCC= 24V) 13
3
(VCC= 20V)
14
Power dissipation a Tamb ≤ 60°C
DIP-8
SO16
Junction and storage temperature -40 to 150 °C
-0.3
-1
-5
6
1
0.8
V
V
V
V
V
W
W
3/13
L4978
Table 6. Electrical Characteristcs
(T
= 25°C, C
j
to T
from 0 to 125°C
j
Symbol Parameter Test Condition Min. Typ. Max. Unit
DYNAMIC CHARACTERISTIC
V
I
V
o
V
d
I
l
f
s
SVRR Supply voltage ripple rejection V
SOFT START
INHIBIT
V
LL
I
sLL
DC CHARACTERISTICS
I
qop
I
q
I
qst-by
ERROR AMPLIFIER
V
FB
R
L
V
oH
V
oL
I
o source
I
o sink
I
b
SVRR E/A Supply voltage ripple rejection V
g
Transconductance I
m
= 2.7nF, R
osc
Operating input voltage range Vo = 3.3 to 50V; Io = 2A
= 20kΩ, VCC = 24V, unless otherwise specified). “●" Specification Referred
osc
●
8 55 V
Output voltage Io = 0.5A 3.33 3.36 3.39 V
I
= 0.2 to 2A
o
= 8 to 55V
V
cc
Dropout voltage V
= 10V; Io = 2A 0.58 0.733 V
cc
Maximum limiting current Vcc = 8 to 55V
Efficiency V
= 3.3V; Io= 2A 87 %
o
Switching frequency
= Vcc+2V
Switching Frequency Stability
vs. V
cc
Temp. stability of switching
i
= 2.5A; f
I
o
Vcc = 8 to 55V 3 6 %
T
= 0 to 125°C 4 %
j
RMS
ripple
; Vo= V
= 100Hz
ref
;
3.292 3.36 3.427 V
●
3.22 3.36 3.5 V
●
●
2.5 3 3.5 A
●
90 100 110 KHz
1.173 V
60 dB
frequency
Soft start charge current 30 40 50 µA
Soft start discharge current 6 10 14 µA
Low level voltage
Isource Low level
Total operating quiescent
●
●
5 15 µA
0.9 V
4 6 mA
current
Quiescent current Duty Cycle = 0; VFB= 3.8V 2.5 3.5 mA
Total stand-by quiescent
current
V
< 0.9V 100 200 µA
inh
= 55V; V
V
cc
<0.9V 150 300 µA
inh
Voltage Feedback Input 3.33 3.36 3.39 V
Line regulation Vcc = 8 to 55V 5 10 mV
Ref. voltage stability vs
●
0.4 mV/°C
temperature
High level output voltage VFB = 2.5V 10.3 V
Low level output voltage VFB = 3.8V 0.65 V
Source output current V
Sink output current V
= 6V; VFB= 2.5V 180 220 µA
comp
= 6V; VFB= 3.8V 200 300 µA
comp
Source bias current 2 3 µA
= VFB; Vcc = 8 to 55V 60 80 dB
comp
DC open loop gain
= ∞
R
L
= -0.1 to 0.1mA
comp
= 6V
V
comp
50 57 dB
2.5 mS
4/13
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