LTC4020_15 Datasheet, PDF (22/42 Pages) Linear Technology – 55V Buck-Boost Multi-Chemistry Battery Charger

LTC4020
APPLICATIONS INFORMATION
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_SENSBOT,
SENSGND
~
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
•
 ^fO 
 250kHz
– 1.0695
600
500
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
,
22
400
300
200
100
0
50 100 150 200 250 300 350 400 450 500
RT (kΩ)
4020 F05
Figure 5. RT vs Operating Frequency
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
4020fb
For more information www.linear.com/LTC4020