LTM8055 datasheet(11/22 Pages) AD

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LTM4693

Rev. 0

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APPLICATIONS INFORMATION

the operating voltage of the application. X5R, X6S or X7R

dielectric types are recommended as they exhibit the best

performance over the wide operating and temperature

ranges. To verify that the intended capacitance is achieved

in the application circuit, be sure to consult the capacitor

vendor’s curve of capacitance vs DC bias voltage.

FORCED CONTINUOUS MODE (FCM)

If the MODE/SYNC pin is high or if the load current on the

converter is high enough to enter forced continuous mode

operation, the LTM4693 operates at a fixed frequency pro-

grammed by the FREQ pin. FCM minimizes output voltage

ripple and yields a low noise switching frequency spec-

trum. A proprietary switching algorithm provides seam-

less transitions between operating modes and eliminates

discontinuities in the average inductor current, inductor

ripple current and loop transfer function throughout all

modes of operation. These advantages result in increased

efficiency, improved loop stability, and lower output volt-

age ripple. In response to the internal control loop com-

mand, an internal pulse width modulator generates the

appropriate switch duty cycle to maintain regulation of

the output voltage.

Burst Mode OPERATION

When the MODE/SYNC pin is held low, the LTM4693 is

configured for Burst Mode operation. As a result, the

buck-boost DC/DC converter will operate with normal

continuous PWM switching above a predetermined aver-

age inductor current and will automatically transition to

power saving Burst Mode operation below this level. With

MODE/SYNC low, at light output loads, the LTM4693 will

go into a standby or sleep state when the output voltage

achieves its nominal regulation level. The sleep state halts

PWM switching and powers down all non-essential func-

tions of the IC, significantly reducing the quiescent cur-

rent of the LTM4693. This greatly improves overall power

conversion efficiency when the output load is light. Since

the converter is not operating in sleep, the output voltage

will slowly decay at a rate determined by the output load

resistance and the output capacitor value. When the out-

put voltage has decayed by a small amount, typically less

than 1%, the LTM4693 will wake up and resume normal

switching operation until the voltage on VOUT is restored

to the previous level. If the load is very light, the LTM4693

may only need to switch for a few cycles to restore VOUT

and may sleep for extended periods of time, significantly

improving efficiency.

AVERAGE CURRENT MODE CONTROL AND STABILITY

COMPENSATION

The LTM4693 utilizes average current mode control for

the pulse width modulator as shown in Figure 3. Current

mode control, both average and the better known peak

methods, enjoy some benefits compared to other con-

trol methods including: simplified loop compensation,

rapid response to load transients and inherent line

voltage rejection.

Referring to Figure 3, an internal high gain transconduc-

tance error amplifier labeled VAMP monitors VOUT through

a voltage divider connected to the FB node and generates

an output, VC, used by the current mode control loop

to command the appropriate inductor current level. To

ensure stability, external frequency compensation com-

ponents (RC and CC) must be installed between VC and

GND and CHF is optional.

VC is internally connected to the non-inverting input of

a second amplifier, referred to in Figure 3 as IAMP. The

inverting input of the average current amplifier is con-

nected to the inductor current sense resistor RCS with a

200mV offset. IAMP contains an internal averaging filter

and frequency compensation network to stabilize opera-

tion of the internal current loop. The average current

amplifier’s output (ICOMP) provides the cycle-by-cycle

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