Datasheet

LTM4602
18
4602fa
Equations for setting frequency for 5V to 3.3V:
I
ON
= (V
IN
– 0.7V)/110k; I
ON
= 39μA
frequency = (I
ON
/[2.4V • 10pF]) • DC = 1.07MHz;
DC = duty cycle, duty cycle is (V
OUT
/V
IN
)
t
S
= t
ON
+ t
OFF
, t
ON
= on-time, t
OFF
= off-time of the
switching period; t
S
= 1/frequency
t
OFF
must be greater than 400ns, or t
S
– t
ON
> 400ns.
t
ON
= DC • t
S
~450kHz frequency or 2.22μs period is chosen for 5V to
3.3V. Frequency range is about 450kHz to 650kHz from
4.5V to 7V input.
t
ON
= 0.66 • 2.22μs ≅ 1.46μs
t
OFF
= 2.22μs – 1.46μs ≅ 760ns
t
ON
and t
OFF
are above the minimums with adequate guard
band.
Using the frequency = (I
ON
/[2.4V • 10pF]) • DC, solve for
I
ON
= (450kHz • 2.4V • 10pF) • (1/0.66) ≅ 16μA. I
ON
current
calculated from 5V input was 39μA, so a resistor from f
ADJ
to ground = (0.7V/30.1k) = 23μA. 39μA – 23μA = 16μA,
sets the adequate I
ON
current for proper frequency range
for the higher duty cycle conversion of 5V to 3.3V. Input
voltage range is limited to 4.5V to 7V. Higher input voltages
can be used without the 30.1k on f
ADJ
. The inductor ripple
current gets too high above 7V, and the 400ns minimum
off-time is limited below 4.5V.
In 12V to 3.3V applications, if a 35k resistor is added from
the f
ADJ
pin to ground, then a 2% effi ciency gain will be
achieved as shown in the 12V effi ciency graph in the Typi-
cal Performance Characteristics. This is due to the lower
transition losses in the power MOSFETs after lowering the
switching frequency down from 1.3MHz to 950kHz.
APPLICATIONS INFORMATION