Datasheet

LTC4020
22
4020fd
For more information www.linear.com/LTC4020
premature triggering by high pass filtering the current sense
signal. Setting the τ of the effective filter in the range of
1ns is generally sufficient to shunt errant signals, such that:
C
SENSB
~
1ns
R
SENSEB
Programming Switching Frequency
The RT frequency adjust pin allows the user to program
the LTC4020 DC/DC converter operating frequency from
50KHz to 500KHz.
Higher frequency operation is desirable for smaller ex
-
ternal inductor and capacitor values, but at the expense
of increased switching losses and higher gate drive cur-
rents. Higher frequencies may also not allow sufficiently
high or low duty cycle operation due to minimum on
/off
time
constraints. Lower operating frequencies require
larger external component values, but result in reduced
switching losses yielding higher conversion efficiencies.
Operating frequency (f
O
) is set by choosing an appropriate
frequency setting resistor (R
RT
), connected from the RT
pin to ground. This resistor is required for operation; do
not leave this pin open. For a desired operating frequency,
R
RT
follows the relation:
R
RT
=100kΩ •
f
O
250kHz




– 1.0695
Input Supply Decoupling
The LTC4020 is typically biased directly from the charger
input supply through the PV
IN
and SENSVIN pins. This
supply provides large switched currents, so a high quality,
low ESR decoupling capacitor is recommended to mini
-
mize voltage glitches on the V
IN
supply. Placing a smaller
ceramic capacitor (0.1µF to 10µF) close to the IC in parallel
with the input decoupling capacitor is also recommended
for high frequency noise reduction. The SENSVIN pin is a
Kelvin connection from the V
IN
supply at the primary V
IN
side switch FET (A); separate decoupling for that pin is
not recommended. The charger input supply decoupling
capacitor (C
VIN
) absorbs all input switching ripple current
in the charger, so it must have an adequate ripple current
rating. RMS ripple current (I
CVIN(RMS)
) is highest during
step down operation, and follows the relation:
I
CVIN(RMS)
~I
MAX
•DC•
1
DC
–
1
,
which has a maximum at DC = 0.5, or V
IN
= 2 • V
OUT
, where:
I
CVIN(RMS)
=
I
MAX
2
The simple worst-case of ½ • I
MAX
is commonly used for
design, where I
MAX
is the programmed inductor current
limit.
Bulk capacitance (C
IN(BULK)
) is a function of desired input
ripple voltage (ΔV
IN
). For step-down operation, C
IN(BULK)
follows the relation:
C
IN(BULK)
≥I
MAX
•
V
OUT(MAX)
V
IN(MIN)
•
1
ΔV
IN
• f
O
,
where f
O
is the operating frequency, V
OUT(MAX)
is the DC/
DC converter maximum output voltage and V
IN(MIN)
is
the regulation voltage corresponding to 2.5V on V
IN_REG
.
If the input regulation feature is not being used, use the
minimum expected input operating voltage.
If an application does not require step-down operation,
during step-up operation, input ripple current is equivalent
to inductor ripple current (ΔI
MAX
), so C
IN(BULK)
follows
the relation:
C
IN(BULK)
=
ΔI
MAX
ΔV
IN
• f
O
Figure 5. RT vs Operating Frequency
applicaTions inForMaTion
RT (kΩ)
50
OPERATING FREQUENCY (kHz)
600
500
400
300
200
100
0
250150 350 400 450
4020 F05
500200100 300
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