LM3370 datasheet(21/33 Pages) TI1

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IRIPPLE

)

= ILOAD + (

)

VIN - VOUT

) * (

2 * L

VOUT

VIN

IMAX = ILOAD + (

LM3370

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SNVS406N – NOVEMBER 2005 – REVISED MAY 2013

Application Information

Setting Output Voltage via I2C-Compatible

The outputs of the LM3370 can be programmed through Buck 1 and Buck 2 registers via I2C. Buck 1 output

voltage can be dynamically adjusted between 1V to 2V in 50 mV steps and Buck 2 output voltage can be

adjusted between 1.8V to 3.3V in 100 mV steps. Finer adjustments to the output of Buck 2 can be achieved with

the placement of a resistor between VOUT2 and the FB2 pin. Typically by placing a 20 K

Ω resistor, R, between

these nodes will result in the programmed Output Voltage increasing by approximately 45 mV,

ΔVTYP.

ΔVTYP= R × 500mV / 234KΩ

(6)

Please refer to for programming the desire output voltage. If the I2C-compatible feature is not used, the default

output voltage will be the pre-trimmed voltage. For example, LM3370SD-3021 refers to 1.2V for Buck 1 and 3.3V

for Buck 2.

VDD Pin

VDD is the power supply to the internal control circuit, if VDD pin is not tied to VIN during normal operating

condition, VDD must be set equal or greater of the two inputs ( VIN1 or VIN2 ). An optional capacitor can be used

for better noise immunity at VDD pin or when VDD is not tied to either VIN pins. Additionally, for reasons of noise

suppression, it is advisable to tie the EN1/EN2 pins to VDD rather than VIN .

SDA, SCL Pins

When not using I2C the SDA and SCL pins should be tied directly to the VDD pin.

Micro-Stepping:

The Micro-Stepping feature minimizes output voltage overshoot/undershoot during large output transients. If

Micro-stepping is enabled through I2C, the output voltage automatically ramps at 50 mV per step for Buck 1 and

100 mV per step for Buck 2. The steps are summarized as follow:

•

Buck 1: 50 mV/step and 32 µs/step

•

Buck 2: 100 mV/step and 32 µs/step

For example if changing Buck 1 voltage from 1V to 1.8V yields 20 steps [(1.8 - 1)/ 0.05 = 20]. This translates to

640

μs [(20 x 32 µs) = 640 µs] needed to reach the final target voltage.

Inductor Selection

There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor

current ripple is small enough to achieve the desired output voltage ripple.

There are two methods to choose the inductor current rating.

method 1:

The total current is the sum of the load and the inductor ripple current. This can be written as

where

•

ILOAD load current

•

VIN input voltage

•

L inductor

•

f switching frequency

(7)

method 2:

A more conservative approach is to choose an inductor that can handle the maximum current limit of 1400 mA.

Given a peak-to-peak current ripple (IPP) the inductor needs to be at least

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