© 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH-EFFICIENCY STEP-DOWN
DC-DC CONVERTER
SC1578
January 29, 1998
FEATURES
• 4V to 24V input voltage operation.
• High efficiency (up to 95%).
• Low quiescent current at 90µA.
• Pulse-skipping and pulse-frequency modulation.
• Inputs-uncommitted current sense comparator.
• Duty cycle adjustable
• 90kHz to 280kHz oscillator frequency
• Power-saving shutdown mode (8µA Typical).
• Push-pull driver output
DESCRIPTION
The SC1578 is a high performance step-down DC-DC
converter, designed to drive an external P-channel
MOSFET to generate programmable output voltages.
Two main schemes of pulse-skipping and pulsefrequency modulation are employed to maintain low
quiescent current and high conversion efficiency over
wide ranges of input voltage and loads. The SC1578
delivers 10mA to 2A of output current with 87%-93%
efficiency at V
IN
=9V, V
OUT
=5V. A current sense
comparator with both inverting and non-inverting inputs
uncommitted is included to provide the crucial function
of either current limit protection or constant output
current control. When the SC1578 is used in a highside current sensing step-down constant current
source, the efficiency is typically greater than 90%.
Duty cycle can be adjusted to greater than 90% by
connecting a resistor from the DUTY pin to V
IN
.
Quiescent current is about 90µA and can be reduced
to 8µA in shutdown mode. With a switching frequency
range of 90kHz to 280kHz, small size switching
components may be used, which is ideal for battery
powered portable equipment.
TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com
BLOCK DIAGRAM
APPLICATIONS
• Notebook 5V/3.3V main power
• Step-down DC-DC converter module.
• Constant current source for battery chargers.
ABSOLUTE MAXIMUM RATINGS
Parameter Symbol Maximum Units
Supply Voltage V
IN
24 V
Duty Voltage V
DUTY
20 V
SHDN Voltage V
SHDN
15 V
Operating
Temperature Range
T
A
0 to 70 °C
Storage Temperature
Range
T
STG
-65 to 150 °C
PIN CONFIGURATION
ORDERING INFORMATION
DEVICE
(1)
PACKAGE
SC1578CS SO-8
Note:
(1) Add suffix ‘TR’ for tape and reel.
© 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH-EFFICIENCY STEP-DOWN
DC-DC CONVERTER
SC1578
January 29, 1998
Parameter Conditions Min Typ Max Units
Operation Voltage 4 24 V
Quiescent Current V
FB
= 1.5V 90 160 µA
Shutdown Mode Current V
SHDN
= 0V 8 20 µA
Internal Reference Voltage 1.16 1.22 1.28 V
Driver Sinking “On resistance” 16
Ω
Driver Sourcing “On resistance” 11
Ω
Current Sense Comparator
Threshold
1578
1578A
V
CS+
= 13V 42
52
57
57
72
62
mV
Shutdown Threshold 0.8 1.5 2.4 V
SHDN Pin Leakage Current V
SHDN
< 15V 1 µA
Duty Cycle V
DUTY
= V
IN
71 %
Oscillator Frequency V
DUTY
= V
IN
225 kHz
TYPICAL PERFORMANCE CHARACTERISTICS
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, TA = 25°C, VIN = 13V
© 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH-EFFICIENCY STEP-DOWN
DC-DC CONVERTER
SC1578
January 29, 1998
Figure 2. 3.3 Step-Down Converter
APPLICATION EXAMPLES
Figure 1. 5V Step-Down Converter
TYPICAL APPLICATION CIRCUIT
to
,
to
© 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH-EFFICIENCY STEP-DOWN
DC-DC CONVERTER
SC1578
January 29, 1998
PIN DESCRIPTIONS
PIN 1: V
IN
4V to 24V input supply voltage.
PIN 5: GND
Power ground.
PIN 2: DUTY
Duty cycle adjustment pin. To be tied
to the V
IN
pin directly or through a resistor to adjust oscillator duty cycle.
See Electrical Characteristics: R
DUTY
must be above 1MΩ if VIN>20V.
PIN 6: DRI
Push-Pull driver output to drive an external P-channel MOSFET or PNP
transistor. When driving a PNP bipolar
transistor, a base resistor and a capacitor to the base of PNP are recommended.
PIN 3: SHDN
Logical input to shutdown the chip:
V
SHDN
= high for normal operation,
V
SHDN
= low for shutdown.
This pin should not be left floating or
forced greater than 15V. In shutdown
mode the DRI pin is high.
PIN 7: CS-
Current sense comparator inverting
input, not to exceed V
IN
.
PIN 4: FB
Feedback comparator input, to compare the feedback voltage with the internal reference voltage. Connecting a
resistor R1 to the converter output and
a resistor R2 to ground yields the output voltage:
V
OUT
= 1.22 x (R1+R2)/R2
PIN 8: CS+
Current sense comparator noninverting input, not to exceed V
IN
.
Figure 3. Battery Charge Circuit with High-Side Current Sensing Constant Current Source
NOTE: R
SENSE
=0.1 ohm, charge current=0.5A ± 10%, VIN>V
BATT
+ 3.5V
R
SENSE
=0.05 ohm, charge current=1A ± 10%, VIN>V
BATT
+ 4V
R
SENSE
=0.033 ohm, charge current=1.5A ± 10%, VIN>V
BATT
+ 4.5V
Efficiency>90%, measured at CS- node
TYPICAL APPLICATION CIRCUIT
© 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH-EFFICIENCY STEP-DOWN
DC-DC CONVERTER
SC1578
January 29, 1998
DEVICE OUTLINE - SO-8
LAND PATTERN - SO-8
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