L4978
2A STEP DOWN SWITCHING REGULATOR
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Features
Figure 1. Packages
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
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.
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 0.25Ω) to obtain very high
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
5
8
R1
20K
L4978
3
C1
220µF
63V
C7
220nF
C2
2.7nF
C5
100nF
2
4
1
7
R2
9.1K
C4
22nF
L1
126µH
(77120)
6
C6
100nF
D1
ST
PS3L60U
VO=3.3V/2A
C8
330µF
D98IN837A
May 2005
Rev. 9
1/13
L4978
Table 2. Block Diagram
VCC
5
THERMAL
SHUTDOWN
VOLTAGES
MONITOR
CBOOT
CHARGE
2
SS_INH
INHIBIT
SOFTSTART
3.3V
INTERNAL
REFERENCE
INTERNAL
SUPPLY
5.1V
7
COMP
6
E/A
8
FB
PWM
R
3.3V
Q
S
CBOOT
CHARGE
AT LIGHT
LOADS
DRIVE
OSCILLATOR
1
3
OSC
BOOT
4
GND
D97IN594
OUT
Figure 3. Pins Connection (Top view)
N.C.
1
16
N.C.
GND
2
15
N.C.
SS_INH
3
14
FB
GND
1
8
FB
SS_INH
2
7
COMP
OSC
4
13
COMP
OSC
3
6
BOOT
OUT
5
12
BOOT
OUT
4
5
VCC
OUT
6
11
VCC
N.C.
7
10
N.C.
N.C.
8
9
N.C.
D97IN595
D97IN596
DIP-8
SO16W
Table 3. Pin Description
N°
Pin
Name
1
2
GND
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
Function
Ground
L4978
Table 3. Pin Description (continued)
N°
Pin
Name
Function
4
5, 6
OUT
Stepdown regulator output
5
11
VCC
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.
(*) 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)
Parameter
Thermal Resistance Junction to ambient
Minidip
SO16
Unit
90 (*)
110 (*)
°C/W
Max.
(*) Package mounted on board.
Table 5. Absolute Maximum Ratings
Symbol
Parameter
Minidip
S016
V5
V11
V4
V5,V6
I4
I5,I6
V6-V5
V12-V11
V6
V12
V7
Value
Unit
Input voltage
58
V
Output DC voltage
Output peak voltage at t = 0.1ms f=200KHz
-1
-5
V
V
Maximum output current
int. limit.
14
V
Bootstrap voltage
70
V
V13
Analogs input voltage (VCC= 24V)
12
V
V2
V3
Analogs input voltage (VCC= 24V)
13
V
V8
V14
(VCC= 20V)
6
-0.3
V
V
1
0.8
W
W
-40 to 150
°C
Ptot
Power dissipation a Tamb ≤ 60°C
Tj,Tstg
Junction and storage temperature
DIP-8
SO16
3/13
L4978
Table 6. Electrical Characteristcs
(Tj = 25°C, Cosc = 2.7nF, Rosc = 20kΩ, VCC = 24V, unless otherwise specified). “● " Specification Referred
to Tj from 0 to 125°C
Symbol
Parameter
Test Condition
Min.
Typ.
Max.
Unit
55
V
DYNAMIC CHARACTERISTIC
VI
Operating input voltage range
Vo = 3.3 to 50V; Io = 2A
Vo
Output voltage
Io = 0.5A
●
Io = 0.2 to 2A
Vcc = 8 to 55V
Vd
Dropout voltage
Vcc = 10V; Io = 2A
Maximum limiting current
Vcc = 8 to 55V
Efficiency
Vo = 3.3V; Io= 2A
●
8
3.33
3.36
3.39
V
3.292
3.36
3.427
V
3.22
3.36
3.5
V
0.58
0.733
V
●
Il
fs
SVRR
●
3
1.173
V
3.5
A
87
●
Switching frequency
2.5
90
100
%
110
60
KHz
dB
Supply voltage ripple rejection
Vi = Vcc+2VRMS; Vo= Vref;
Io = 2.5A; f ripple= 100Hz
Switching Frequency Stability
vs. Vcc
Vcc = 8 to 55V
3
Temp. stability of switching
frequency
Tj = 0 to 125°C
4
6
%
%
SOFT START
Soft start charge current
30
40
50
µA
Soft start discharge current
6
10
14
µA
INHIBIT
VLL
Low level voltage
●
IsLL
Isource Low level
●
0.9
V
5
15
µA
4
6
mA
DC CHARACTERISTICS
Iqop
Iq
Iqst-by
Total operating quiescent
current
Quiescent current
Duty Cycle = 0; VFB= 3.8V
2.5
3.5
mA
Total stand-by quiescent
current
Vinh < 0.9V
100
200
µA
Vcc = 55V; Vinh
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