LT8610AC datasheet(13/26 Pages) LINER

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LT8610AC/LT8610AC-1

8610acfa

For more information www.linear.com/LT8610AC

applicaTions inForMaTion

programmed frequency varies based on input voltage,

output voltage, and inductor choice.

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. If higher efficiency is

needed in a Burst Mode application, increasing inductor

value can be a quick solution.

Table 1. Output Voltage Ripple vs Output Capacitance for

LT8610AC when VIN = 12V, VOUT = 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 LT8610AC/

LT8610AC-1 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 cur-

rent 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 INTVCC pin. When a clock is applied to the SYNC

pin the LT8610AC/LT8610AC-1 will also operate in pulse-

skipping mode.

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:

=R2

VOUT

0.80V

–1









(1)

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy.

Iflowinputquiescentcurrentandgoodlight-loadefficiency

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:

IQ =1.7µA+

VOUT

+R2









VOUT

VIN

















(2)

where 1.7µA is the quiescent current of the LT8610AC/

LT8610AC-1 and the second term is the current in the

feedbackdividerreflectedtotheinputofthebuckoperating

at its light load efficiency n. For a 3.3V application with R1

= 1M and R2 = 316k, the feedback divider draws 2.5µA.

With VIN = 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 VIN; this is plotted in the

Typical Performance Characteristics section.

When using large FB resistors, a 4.7pF to 10pF phase-lead

capacitor should be connected from VOUT to FB.

Setting the Switching Frequency

The LT8610AC uses a constant frequency PWM architec-

ture 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 RT value for a desired

switching frequency is in Table 2A. The LT8610AC-1 can

be programmed to switch from 1.5MHz to 2.2MHz. The

minimum allowed programmable switching frequency is

higher for the LT8610AC-1 compared to the LT8610AC

becausetheLT8610AC-1hasinternalcompensationwhich

is optimized for higher switching frequencies to improve

transient response by increasing control loop bandwidth.

A table showing the necessary RT value for a desired

switching frequency is shown in Table 2B.

The RT resistor required for a desired switching frequency

can be calculated using:

RT =

46.5

fSW

– 5.2

(3)

where RT is in kΩ and fSW is the desired switching fre-

quency in MHz.

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