LTM4600HV
18
4600hvfe
For more information www.linear.com/LTM4600HV
Figure 20. Recommended PCB Layout
V
IN
PGND
TOP LAYER
V
OUT
4600hv F20
LOAD
C
IN
LTM4600HV Frequency Adjustment
The LTM4600HV is designed to typically operate at 850kHz
across most input and output conditions. The control ar
-
chitecture is constant on time valley mode current control.
The f
ADJ
pin is typically left open or decoupled with an
optional 1000pF capacitor. The switching frequency has
been optimized to maintain constant output ripple over the
operating conditions. The equations for setting the operat
-
ing frequency
are set around a programmable constant on
time. This on time is developed by a programmable current
into an on board 10pF capacitor that establishes a ramp
that is compared to a voltage threshold equal to the output
voltage up to a 2.4V clamp. This I
ON
current is equal to:
I
ON
= (V
IN
– 0.7V)/110k, with the 110k onboard resistor
from V
IN
to f
ADJ
. The on time is equal to t
ON
= (V
OUT
/I
ON
)
•10pF and t
OFF
= t
s
– t
ON
. The frequency is equal to: Freq.
= DC/t
ON
. The I
ON
current is proportional to V
IN
, and the
regulator duty cycle is inversely proportional to V
IN
, there-
fore the step
-down regulator will remain relatively constant
frequency as the duty cycle adjustment takes place with
lowering
V
IN
. The on time is proportional to V
OUT
up to a
2.4V clamp. This will hold frequency relatively constant
with different output voltages up to 2.4V. The regulator
switching period is comprised of the on time and off time
as depicted in the following waveform. The on time is
equal to t
ON
= (V
OUT
/I
ON
)•10pF and t
OFF
= t
s
– t
ON
. The
frequency is equal to: Frequency = DC/t
ON
).
The LTM4600HV has a minimum (t
ON
) on time of 100
nanoseconds and a minimum (t
OFF
) off time of 400
nanoseconds. The 2.4V clamp on the ramp threshold as
a function of V
OUT
will cause the switching frequency to
increase by the ratio of V
OUT
/2.4V for 3.3V and 5V outputs.
This is due to the fact the on time will not increase as V
OUT
increases past 2.4V. Therefore, if the nominal switch-
ing frequency
is 850kHz, then the switching frequency
will increase to ~1.2MHz for 3.3V, and ~1.7MHz for 5V
outputs due to Frequency = (DC/t
ON
) When the switching
frequency increases to 1.2MHz, then the time period t
S
is
reduced to ~833 nanoseconds and at 1.7MHz the switching
period reduces to ~588 nanoseconds. When higher duty
cycle conversions
like 5V to 3.3V and 12V to 5V need to
be accommodated, then the switching frequency can be
lowered to alleviate the violation of the 400ns minimum
off time. Since the total switching period is t
S
= t
ON
+ t
OFF
,
t
OFF
will be below the 400ns minimum off time. A resistor
from the f
ADJ
pin to ground can shunt current away from
the on time generator, thus allowing for a longer on time
and a lower switching frequency. 12V to 5V and 5V to
3.3V derivations are explained in the data sheet to lower
switching frequency and accommodate these step-down
conversions.
Equations for setting frequency for 12V to 5V:
I
ON
= (V
IN
– 0.7V)/110k; I
ON
= 103µA
frequency = (I
ON
/[2.4V • 10pF]) • DC = 1.79MHz;
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
1MHz frequency or 1µs period is chosen for 12V to 5V.
t
OFF
PERIOD t
s
t
ON
4602 F25
(DC) DUTY CYCLE =
ON
t
s
DC = =
t
ON
t
s
FREQ =
DC
t
ON
V
OUT
V
IN
applicaTions inForMaTion
