LT1976IFE Linear Technology, LT1976IFE Datasheet - Page 11

• Manufacturer Part Number: LT1976IFE
• Description: IC REG SW STEP-DWN 1.5A 16-TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1976IFE

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.2 ~ 54 V
• Current - Output: 1.5A
• Frequency - Switching: 200kHz
• Voltage - Input: 3.3 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
BLOCK DIAGRA
power from the V
an external voltage higher than 3V bias power will be
drawn from the external source (typically the regulated
output voltage). This improves efficiency.
High switch efficiency is attained by using the BOOST pin
to provide a voltage to the switch driver which is higher
than the input voltage, allowing switch to be saturated.
This boosted voltage is generated with an external capaci-
tor and diode.
To further optimize efficiency, the LT1976 automatically
switches to Burst Mode operation in light load situations.
CHOOSING THE LT1976 OR LT1977
The LT1976/LT1976B and LT1977 are high voltage 1.5A
step-down switching regulators. The LT1976 and LT1977
contain circuitry which shifts into burst mode at light loads
reducing quiescent current to typically 100μA. The LT1976B
pulse skips in light load situations, eliminating low fre-
quency burst mode output ripple at the expense of light
load efficiency. The difference between the LT1976/
LT1976B and LT1977 is that the fixed switching frequency
of the LT1976/LT1976B is 200kHz versus 500kHz for the
LT1977. The switching frequency affects: inductor size,
input voltage range in continuous mode operation, effi-
ciency, thermal loss and EMI.
OUTPUT RIPPLE AND INDUCTOR SIZE
Output ripple current is determined by the input to output
voltage ratio, inductor value and switch frequency. Since
the switch frequency of the LT1977 is 2.5 times greater
than that of the LT1976/LT1976B, the inductance used in
the LT1977 application can be 2.5 times lower than the
LT1976/LT1976B while maintaining the same output ripple
current. The lower value used in the LT1977 application
allows the use of a physically smaller inductor.
IN
U
pin, but if the BIAS pin is connected to
U
W
W
U
INPUT VOLTAGE RANGE
The minimum on and off times for all versions of the ic are
equivalent. This results in a narrower range of continuous
mode operation for the LT1977. Typical minimum and
maximum duty cycles are 6% to 92% for the LT1976/
LT1976B and 15% to 90% for the LT1977. Both parts will
regulate up to an input voltage of 60V but the LT1977 will
transistion into pulse-skipping/Burst Mode operation when
the input voltage is above 30V for a 5V output. At outputs
above 10V the LT1977’s input range will be similar to the
LT1976/LT1976B. Lowering the input voltage below the
maximum duty cycle limitation will cause a dropout in
regulation.
Table 1. LT1976/LT1976B/LT1977 Comparison
In Burst Mode operation, all circuitry associated with
controlling the output switch is shut down reducing the
input supply current to 45μA.
The only difference between the LT1976 and the LT1976B
is that the LT1976B does not shift into burst mode in light
load situations, eliminating low frequency output ripple at
the expense of light load efficiency.
The LT1976 contains a power good flag with a program-
mable threshold and delay time. A logic-level low on the
SHDN pin disables the IC and reduces input suppy current
to less than 1μA.
Light Load Output Ripple
Output Capacitor Size
Maximum Duty Cycle
Minimum Duty Cycle
Output Ripple
PARAMETER
Inductor Size
Input Range
Efficiency
EMI
LT1976/LT1976B
LT1976/LT1976B
LT1976/LT1976B
LT1976/LT1976B
ADVANTAGE
LT1976B
LT1976B
LT1977
LT1977
LT1976
LT1977
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