LTC3879_15 Datasheet, PDF (13/28 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down Controller

LTC3879
APPLICATIONS INFORMATION
Figure 3 shows how R_ON relates to switching frequency
for several common output voltages.
When designing for pseudo fixed frequency, there is sys-
tematic error because the I_ON pin voltage is approximately
0.7V, not zero. This causes the I_ON current to be inversely
proportional to (V_IN – 0.7V) and not V_IN. The I_ON current
error increases as V_IN decreases. To correct this error, an
additional resistor R_ON2 can be connected from the I_ON
pin to the 5.3V INTV_CC supply.
R_ON2
=
5.3V – 0.7V
0.7V
R_ON
1000
V_OUT = 12V
V_OUT = 1.5V
V_OUT = 3.3V
V_OUT = 5V
100
100
1000
R_ON (kΩ)
10000
3879 F03
Figure 3. Switching Frequency vs R_ON
Minimum Off-Time and Dropout Operation
The minimum off-time, t_OFF(MIN), is the shortest time
required for the LTC3879 to turn on the bottom MOSFET,
trip the current comparator and then turn off the bottom
MOSFET. This time is typically about 220ns. The minimum
off-time limit imposes a maximum duty cycle of t_ON/
(t_ON + t_OFF(MIN)). If the maximum duty cycle is reached,
due to a drooping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
V_IN(MIN)
=
V_OUT
t_ON
+ t_OFF(MIN)
t_ON
A plot of maximum duty cycle vs. frequency is shown in
Figure 4.
4
3
DROPOUT
REGION
2
1
0
0
0.25
0.50
0.75
1
DUTY CYCLE (V_OUT/V_IN)
3879 F04
Figure 4. Maximum Switching Frequency vs Duty Cycle
Likewise, the maximum frequency of operation is deter-
mined by the fixed on-time, t_ON, and the minimum off-time,
t_OFF(MIN). The fixed on-time is determined by dividing the
duty factor by the nominal frequency of operation:
f_MAX
=
V_OUT
V_IN • f_OP
1
+ t_OFF(MIN)
[Hz]
The LTC3879 is a PFM (pulse frequency mode) regula-
tor where pulse density is modulated, not pulse width.
Consequently, frequency increases with a load step and
decreases with a load release. The steady-state operating
frequency, f_OP, should be set sufficiently below f_MAX to
allow for device tolerances and transient response.
Inductor Value Calculation
Given the desired input and output voltages, the induc-
tor value and operation frequency determine the ripple
current:
ΔI_L
=
⎛
⎝⎜
V_OUT ⎞
f_OP • L _⎠⎟
⎛
⎝⎜ ^1–
V_OUT
V_IN
⎞
⎠⎟
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
3879f
13