Datasheet

LTM4600HV
18
4600hvfc
APPLICATIO S I FOR ATIO
WUU
U
t
ON
= 0.41 • 1μs ≅ 410ns
t
OFF
= 1μs – 410ns ≅ 590ns
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
= (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 9V to 16V. Higher
input voltages can be used without the 15k on f
ADJ
. The
inductor ripple current gets too high above 16V, and the
400ns minimum off-time is limited below 9V.
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.
4600 F22
R2
22.1k
1%
R1
30.1k
EXTV
CC
RUN/SS
COMP
FCB
V
OUT
5V TO 3.3V AT 8A WITH f
ADJ
= 30.1k
LTM4600HV MINIMUM ON-TIME = 100ns
LTM4600HV MINIMUM OFF-TIME = 400ns
C1, C3: TDK C3216X5R1E106MT
C2: TAIYO YUDEN, JMK316BJ226ML
C4: SANYO POSCAP, 6TPE330MIL
PGOOD
V
OSET
SV
IN
PGNDSGND
4.5V TO 7V
3.3V AT 8A
C1
10μF
25V
C3
10μF
25V
C4
330μF
6.3V
C2
22μF
C5
100pF
V
IN
LTM4600HV
f
ADJ
RUN/SOFT-START
OPEN DRAIN
EFFICIENCY = 94%
+
5V to 3.3V at 8A