LT8331 datasheet(15/24 Pages) AD

15 / 24 page

LT8337/LT8337-1

Rev. 0

For more information www.analog.com

FB Resistor Network and the Quiescent Current at

No Load

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the resistor

values according to Equation 2.

R1 = R2 •

VOUT

– 1

⎛

⎝⎜

⎞

⎠⎟

(2)

Reference designators refer to Block Diagram. The

1% resistors are recommended to maintain output

voltage accuracy.

If low input quiescent current and good light-load effi-

ciency are desired, use large resistor values for the FB

resistor divider. The current flowing in the divider acts as

a load current, and will increase the no load input current

to the converter.

When VIN < VOUT, the LT8337 converter Burst Mode quies-

cent current at no load can be estimated using Equation 3,

and the LT8337-1 converter Burst Mode quiescent current

at no load can be estimated using Equation 4.

I ≈

VOUT

R1 + R2

4µA

1µA

VOUT

VIN

1.2

•

⎛

⎝

⎜

⎛

⎝

⎜

⎞

⎠

⎟

⎞

⎠

⎟

(3)

I ≈

VOUT

R1 + R2

23µA

1µA

VOUT

VIN

1.4

•

⎛

⎝

⎜

⎛

⎝

⎜

⎞

⎠

⎟

⎞

⎠

⎟

(4)

where 4µA and 23µA are the VIN pin quiescent current of

the LT8337 and LT8337-1 respectively, and the second

term is the current drawn by the feedback divider and VOUT

pin (1μA) reflected to the input of the boost operating.

For a 12V input, 24V output boost converter with R1 = 1M

and R2 = 43.2k, it can be calculated that the LT8337 con-

verter draws approximately 60µA from the supply at no

load, and the LT8337-1 converter draws approximately

90µA from the supply at no load. Note that Equation 3

and Equation 4 imply that the no load current is a func-

tion of VIN.

When VIN is higher than the regulated VOUT voltage, the IC

enters PassThru operation and VOUT is essentially shorted

to VIN by the inductor and M2. The converter quiescent

current at no load can be estimated using Equation 5 for

LT8337 and Equation 6 for LT8337-1.

I ≈

VIN

R1 + R2

45µA +

(5)

I ≈

VIN

R1 + R2

60µA +

(6)

where 45µA and 60µA are is the sum of the VIN pin and

VOUT pin quiescent current of the LT8337 and LT8337-1

respectively, and the second term is the current drawn by

the feedback divider.

When using large FB resistors, a 4.7pF to 22pF phase-lead

capacitor should be connected from VOUT to FB, and a

careful evaluation of system stability should be made to

ensure adequate design margin.

Overvoltage Lockout

The VOUT pin voltage is constantly monitored by the

LT8337/LT8337-1. An overvoltage condition occurs when

VOUT pin voltage exceeds approximately 28V. Switching is

stopped at such condition. Normal switching is resumed

when the VOUT pin voltage drops back to 28V or lower.

Switching Frequency Foldback when VIN

Approaches VOUT

In some applications, VIN may rise to a voltage very close

to VOUT. In this condition the switching regulator must

operate at a very low duty cycles to keep VOUT in regula-

tion. However, the minimum on-time limitation may pre-

vent the switcher from attaining a sufficiently low duty

cycle at the programmed switching frequency. As a result

a typical boost converter may experience a large output

ripple under these conditions. The LT8337/LT8337-1

addresses this issue by adopting a switching frequency

foldback function to smoothly decrease the switching

frequency when its minimum on-time starts to limit the

APPLICATIONS INFORMATION

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