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LT8331 Datasheet(HTML) 9 Page - Analog Devices

Part No. LT8331
Description  Low IQ Boost/SEPIC/Flyback/Inverting Converter with 0.5A, 140V Switch
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LT8331 Datasheet(HTML) 9 Page - Analog Devices

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LT8331
9
Rev. C
For more information www.analog.com
APPLICATIONS INFORMATION
Figure 1. Burst Frequency vs Load Current
ACHIEVING ULTRALOW QUIESCENT CURRENT
To enhance efficiency at light loads the LT8331 uses a low
ripple Burst Mode architecture. This keeps the output capaci-
tor charged to the desired output voltage while minimizing
the input quiescent current and output ripple. In Burst Mode
operation, the LT8331 delivers single small pulses of current
to the output capacitor followed by sleep periods where the
output power is supplied by the output capacitor. While in
sleep mode, the LT8331 consumes only 6µA.
As the output load decreases, the frequency of single current
pulses decreases (see Figure 1) and the percentage of time
the LT8331 is in sleep mode increases, resulting in much
higher light load efficiency than for typical converters. To
optimize the quiescent current performance at light loads,
the current in the feedback resistor divider must be mini-
mized as it appears to the output as load current. In addition,
all possible leakage currents from the output should also
be minimized as they all add to the equivalent output load.
The largest contributor to leakage current can be due to the
reverse biased leakage of the Schottky diode (see Diode
Selection in the Applications Information section).
While in Burst Mode operation, the current limit of the
switch is approximately 140mA resulting in the output
voltage ripple shown in Figure 2. Increasing the output
capacitance will decrease the output ripple proportionally.
As the output load ramps upward from zero the switching
frequency will increase but only up to the fixed frequency
defined by the resistor at the RT pin as shown in Figure 1.
The output load at which the LT8331 reaches the fixed
Figure 2. Burst Mode Operation
frequency varies based on input voltage, output voltage,
and inductor choice.
PROGRAMMING INPUT TURN-ON AND TURN-OFF
THRESHOLDS WITH EN/UVLO PIN
The EN/UVLO pin voltage controls whether the LT8331 is
enabled or is in a shutdown state. A 1.6V reference and
a comparator A6 with built-in hysteresis (typical 140mV)
allow the user to accurately program the system input
voltage at which the IC turns on and off (see the Block
Diagram). The typical input falling and rising threshold
voltages can be calculated by the following equations:
VIN(FALLING,UVLO(–)) = 1.60 •
R3 + R4
R4
VIN(RISING, UVLO(+)) = 1.74 •
R3 + R4
R4
VIN current is reduced below 1µA when the EN/UVLO pin
voltage is less than 0.2V. The EN/UVLO pin can be con-
nected directly to the input supply VIN for always-enabled
operation. A logic input can also control the EN/UVLO pin.
When operating in Burst Mode operation for light load
currents, the current through the R3 and R4 network can
easily be greater than the supply current consumed by the
LT8331. Therefore, R3 and R4 should be large enough to
minimize their effect on efficiency at light loads.
INTVCC REGULATOR
A low dropout (LDO) linear regulator, supplied from VIN,
produces a 3.2V supply at the INTVCC pin. A minimum
1µF low ESR ceramic capacitor must be used to bypass
the INTVCC pin to ground to supply the high transient cur-
rents required by the internal power MOSFET gate driver.
FRONT PAGE APPLICATION
VIN = 48V, VOUT = 48V
LOAD CURRENT (mA)
0
20
40
60
80
100
0
150
300
450
600
8331 F01
5µs/DIV
VOUT
20mV/DIV
IL1+IL2
100mA/DIV
8331 F02


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