LT8331 datasheet(9/24 Pages) AD

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LT8334

Rev. 0

For more information www.analog.com

ACHIEVING ULTRALOW QUIESCENT CURRENT

To enhance efficiency at light loads, the LT8334 uses a

low ripple Burst Mode architecture. This keeps the out-

put capacitor charged to the desired output voltage while

minimizing the input quiescent current and output ripple.

In Burst Mode operation, the LT8334 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 LT8334 consumes

only 9µA.

As the output load decreases, the frequency of single cur-

rent pulses decreases (see Figure 1) and the percentage

of time the LT8334 is in sleep mode increases, resulting

in much higher light load efficiency than for typical con-

verters. To optimize the quiescent current performance

at light loads, the current in the feedback resistor divider

must be minimized 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).

FRONT PAGE APPLICATION

VIN = 6V

VOUT = 24V

LOAD CURRENT(mA)

0 10 20 30 40 50 60 70 80 90 100

0.5

1.0

1.5

2.0

2.5

8334 F01

Figure 1. Burst Frequency vs Load Current

While in Burst Mode operation, the current limit of the

switch is approximately 1.25A 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

APPLICATIONS INFORMATION

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 LT8334 reaches the fixed

frequency varies based on input voltage, output voltage,

and inductor choice.

10µs/DIV

1A/DIV

VOUT

5mV/DIV

8334 F02

Figure 2. Burst Mode Operation

PROGRAMMING INPUT TURN-ON AND TURN-OFF

THRESHOLDS WITH EN/UVLO PIN

The EN/UVLO pin voltage controls whether the LT8334 is

enabled or is in a shutdown state. A 1.6V reference and

a comparator A6 with built-in hysteresis (typical 80mV)

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 input-rising thresh-

old voltages can be calculated with Equation 1.

VIN(FALLING,UVLO(–)) = 1.60 •

R3 + R4

VIN(RISING, UVLO(+)) = 1.68 •

R3 + R4

(1)

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

LT8334. Therefore, R3 and R4 should be large enough to

minimize their effect on efficiency at light loads.

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