Datasheet

LTM4602HV
18
4602hvf
APPLICATIO S I FOR ATIO
WUU
U
Using the frequency = (I
ON
/[2.4V • 10pF]) • (DC), solve for
I
ON
= (1MHz • 2.4V • 10pF) • (1/0.41) ≅ 58µA. I
ON
current
calculated from 12V input was 103µA, so a resistor from
f
ADJ
to ground = (0.7V/15k) = 46µA. 103µA – 46µA =
57µA, sets the adequate I
ON
current for proper frequency
range for the higher duty cycle conversion of 12V to
5V. Input voltage range is limited to 8V to 16V. Higher
input voltages can be used without the 15k on f
ADJ
.
The inductor ripple current gets too high above 16V or
below 8V.
Equations for setting frequency: V
OUT
= 3.3V
I
ON
= (V
IN
– 0.7V)/110k; for 5V input, I
ON
= 39µA
frequency = (I
ON
/[2.4V • 10pF]) • (DC) = 1.07MHz;
DC = duty cycle, duty cycle is (V
OUT
/V
IN
)
t = t
ON
+ t
OFF
, t
ON
= on-time, t
OFF
= off-time of the
switching period; t = 1/frequency
t
OFF
must be greater than 400ns, or t – t
ON
> 400ns.
t
ON
= DC • t
~450kHz frequency or 2.22µs period is chosen. 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 induc-
tor ripple current gets too high above 7V, and the 400ns
minimum off-time is limited below 4.5V.
Therefore, at 3.3V output, a 30.1k resistor is recommended
to add from pin f
ADJ
to ground when the input voltage is
between 4.5V to 7V. However, this resistor needs to be
removed to avoid high inductor ripple current when the
input voltage is more than 7V. Similarly, for 5V output, a
15k resistor is recommended to adjust the frequency when
the input voltage is between 8V to 16V. This 15k resistor
is removed when the input voltage becomes higher than
16V. Please refer to the Typical Performance curve V
IN
to
V
OUT
Step-Down Ratio.
In 12V to 3.3V and 24V 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 and
24V effi ciency graphs shown in the Typical Characteris-
tics. This is due to lowering the transition losses in the
power MOSFETs by reducing the switching frequency
from 1.3mHz to 1mHz.
5V to 3.3V at 5A
4602HV F23
R
SET
22.1k
1%
R1
30.1k
EXTV
CC
RUN/SS
COMP
FCB
V
OUT
5V TO 3.3V AT 5A WITH f
ADJ
= 30.1k
LTM4602HV MINIMUM ON-TIME = 100ns
LTM4602HV MINIMUM OFF-TIME = 400ns
C1, C3: TDK C3216X5R1E106MT
C2: TAIYO YUDEN, JMK316BJ226ML
C4: SANYO POS CAP, 6TPE330MIL
PGOOD
V
OSET
SV
IN
PGNDSGND
V
IN
4.5V TO 7V
V
OUT
3.3V AT 5A
C1
10µF
25V
C3
10µF
25V
C4
330µF
6.3V
C2
22µF
C5
100pF
V
IN
LTM4602HV
f
ADJ
RUN/SOFT-START
OPEN DRAIN
EFFICIENCY = 92%
+