LTM8080 datasheet(17/32 Pages) AD

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LTM4703

Rev. 0

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railed at its maximum value (when VSET has risen sig-

nificantly about VOSNS). This prevents incorrect behavior

where the 2.7mA current sources stays on even if VSET

has risen above its intended value.

Also, there is a minimum SET capacitor requirement for

using a fast start-up without overcharging the reference

voltage. This depends on the compensation network, as

the part is depending on the COMPa pin voltage rising to

its maximum value to inform the part to pause fast charge.

Due to the high gm of the error amplifier, the capacitor in

the compensation network typically dominates the COMPa

voltage rise time. In this case, the minimum required SET

capacitance value to prevent overcharging the reference

voltage is given by Equation 5.

Minimum CSET = 27•

CCOMP

VSET

(5)

If programmable power good and fast start-up capabili-

ties are not required, the PGSET pin must be connected

to 0.5V.

Prebiased Output

The LTM4703 regulates the output to the VSET voltage.

If the LTM4703 output is higher than the target output

voltage, the LTM4703 attempts to regulate the output to

the target voltage by returning a small amount of energy

back to the input supply. If nothing is loading the input

supply, its voltage may rise. Ensure that it does not rise

too high and that the input voltage exceeds the absolute

maximum rating of the LTM4703.

Forced Continuous Mode (FCM)

The LTM4703 can operate in FCM for fast transient

response and full-frequency operation over a wide load

range. When in FCM, the oscillator operates continuously,

and positive SW transitions are aligned to the clock. A

negative inductor current is allowed at light loads or under

large transient conditions. The LTM4703 can sink current

from the output and return this charge to the input in

this mode, improving load step transient response (see

Figure 1). At light loads, FCM operation is less efficient

than PSM operation, but it may be desirable in applications

where it is necessary to keep switching harmonics out

of the signal band. The FCM must be used if the output

is required to sink current. To enable FCM, connect the

SYNC/MODE pin to INTVCC or > 1.5V, or float the pin.

PVIN = 12V, VOUT = 1V, fSW = 2MHz

CC = 2.2nF, RC = 1k

COUT = 100µF ×2 + 22µF ×2

PSM

FCM

20µs/DIV

ILOAD

2A/DIV

VOUT

50mV/DIV

4703 F01

Figure 1. Load Step Transient Response with and

without FCM

The FCM is disabled:

1) Under PVIN overvoltage conditions (PVIN pin is held

above 16V),

2) If VOUT is too high (the PGSET pin is held greater

than 537.5mV), and

3) During start-up until the voltage on VOUT has charged

up to 92.5% of its final value (as indicated when the

PGSET pin rises to above 462.5mV).

For the latter two conditions, it is assumed the PGSET pin

is connected to the output voltage through an appropriate

resistor. When FCM is disabled in these ways, the neg-

ative inductor current is not allowed, and the LTM4703

operates in PSM.

Pulse-Skipping Mode (PSM)

When not operating in FCM, the LTM4703 operates

in PSM. In this mode, the oscillator operates continu-

ously, and all switching cycles are aligned to the clock.

A negative inductor current is not allowed in this mode;

therefore, at light loads, the LTM4703 may be operat-

ing in discontinuous mode. Additionally, in PSM, the

LTM4703 may also skip switching cycles at very light

loads for improved efficiency, or at very high duty cycles

to achieve better dropout. To enable PSM, connect the

SYNC pin to GND.

APPLICATIONS INFORMATION

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