LT8610A_15 Datasheet, PDF (14/26 Pages) Linear Technology – 42V, 3.5A Synchronous Step-Down Regulator with 2.5A Quiescent Current

LT8610A/LT8610AB Series
APPLICATIONS INFORMATION
which the LT8610A/LT8610AB reaches the programmed
frequency varies based on input voltage, output voltage,
and inductor choice. However, the output load required
to reach full frequency will be higher for the LT8610AB
as compared to the LT8610A (Figure 1a).
Inductor value has a very strong effect on Burst Mode ef-
ficiency. Larger value inductors allow more charge to be
transferred to the output per pulse, which increases both
efficiency and output voltage ripple. This dependence on
inductance is stronger for the LT8610AB than it is for the
LT8610A. If higher efficiency is needed in a Burst Mode ap-
plication, increasing inductor value can be a quick solution.
Table 1. Output Voltage Ripple vs Output Capacitance for
LT8610AB when V_IN = 12V, V_OUT = 3.3V, and L = 4.7µH
OUTPUT CAPACITANCE
OUTPUT RIPPLE
47µF
40mV
47µF ×2
20mV
47µF ×4
10mV
For some applications it is desirable for the LT8610A/
LT8610AB to operate in pulse-skipping mode, offering
two major differences from Burst Mode operation. First
is the clock stays awake at all times and all switching
cycles are aligned to the clock. In this mode much of
the internal circuitry is awake at all times, increasing
quiescent current to several hundred µA. Second is that
full switching frequency is reached at lower output load
than in Burst Mode operation (see Figure 1b). To enable
pulse-skipping mode, the SYNC pin is tied high either to a
logic output or to the INTV_CC pin. When a clock is applied
to the SYNC pin the LT8610A/LT8610AB will also operate
in pulse-skipping mode.
When using large FB resistors, a 4.7pF to 10pF phase-lead
capacitor should be connected from V_OUT to FB.
The fixed output versions of the LT8610A/LT8610AB
series have the feedback resistor network and phase
lead capacitor integrated within the part. The FB pin is
replaced with a V_OUT pin for these regulators. The V_OUT
pin can be connected directly to the inductor and output
capacitor. The 3.3V fixed output products (LT8610A-3.3/
LT8610AB-3.3) have a total of 14.3M of internal feedback
divider resistance from the V_OUT pin to ground. The 5V
fixed output products (LT8610A-5/LT8610AB-5) have a
total of 12.5M of internal feedback divider resistance from
the V_OUT pin to ground.
If low input quiescent current and good light-load efficiency
are desired, use large resistor values for the FB resistor
divider. The current flowing in the divider acts as a load
current, and will increase the no-load input current to the
converter, which is approximately:
I_Q
=
1.7µA
+


V_OUT
R1+ R2




V_OUT
V_IN




1
n ^
(2)
where 1.7µA is the quiescent current of the LT8610A/
LT8610AB and the second term is the current in the feed-
back divider reflected to the input of the buck operating at
its light load efficiency n. For a 3.3V application with R1
= 1M and R2 = 412k, the feedback divider draws 2.3µA.
With V_IN = 12V and n = 80%, this adds 0.8µA to the 1.7µA
quiescent current resulting in 2.5µA no-load current from
the 12V supply. Note that this equation implies that the
no-load current is a function of V_IN; this is plotted in the
Typical Performance Characteristics section.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1=
R2


V_OUT
0.970V
–
1^{}
(1)
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
Setting the Switching Frequency
The LT8610A/LT8610AB uses a constant frequency PWM
architecture that can be programmed to switch from
200kHz to 2.2MHz by using a resistor tied from the RT
pin to ground. A table showing the necessary R_T value for
a desired switching frequency is in Table 1.
The R_T resistor required for a desired switching frequency
can be calculated using:
R_T
=
46.5
f_SW
–
5.2
(3)
8610abfa
14
For more information www.linear.com/LT8610A