Datasheet
I
RIPPLE
)
2
= I
LOAD
+ (
1
f
)
V
IN
- V
OUT
) * ( ) * (
2 * L
V
OUT
V
IN
I
MAX
= I
LOAD
+ (
LM3370
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SNVS406N –NOVEMBER 2005–REVISED MAY 2013
Application Information
Setting Output Voltage via I
2
C-Compatible
The outputs of the LM3370 can be programmed through Buck 1 and Buck 2 registers via I
2
C. 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 KΩ resistor, R, between
these nodes will result in the programmed Output Voltage increasing by approximately 45 mV,ΔV
TYP
.
ΔV
TYP
= R × 500mV / 234KΩ (6)
Please refer to for programming the desire output voltage. If the I
2
C-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.
V
DD
Pin
V
DD
is the power supply to the internal control circuit, if V
DD
pin is not tied to V
IN
during normal operating
condition, V
DD
must be set equal or greater of the two inputs ( V
IN1
or V
IN2
). An optional capacitor can be used
for better noise immunity at V
DD
pin or when V
DD
is not tied to either V
IN
pins. Additionally, for reasons of noise
suppression, it is advisable to tie the EN1/EN2 pins to V
DD
rather than V
IN
.
SDA, SCL Pins
When not using I
2
C the SDA and SCL pins should be tied directly to the V
DD
pin.
Micro-Stepping:
The Micro-Stepping feature minimizes output voltage overshoot/undershoot during large output transients. If
Micro-stepping is enabled through I
2
C, 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
• I
LOAD
load current
• V
IN
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 (I
PP
) the inductor needs to be at least
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