LTC3370 datasheet(17/26 Pages) AD

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LTC3307A

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

Transient performance and control loop stability can be

improved with a higher COUT and/or the addition of a

feedforward capacitor CFF placed between VOUT and FB.

Capacitor CFF provides phase lead compensation by cre-

ating a high frequency zero which improves the phase

margin and the high-frequency response. The values used

in the typical application circuits are a good starting point.

LTpowerCAD® is a useful tool to help optimize CFF and

COUT for a desired transient performance.

Applying a load transient and monitoring the response of

the system or using a network analyzer to measure the

actual loop response are two ways to experimentally verify

transient performance and control loop stability, and to

optimize CFF and COUT.

When using the load transient response method to sta-

bilize the control loop apply an output current pulse of

20% to 100% of full load current having a very fast rise

time. This will produce a transient on the output voltage.

Monitor VOUT for overshoot or ringing that might indicate

a stability problem (see Application Note AN149).

Output Voltage Sensing

The LTC3307A’s AGND pin is the ground reference for the

internal analog circuitry, including the bandgap voltage

reference. To achieve good load regulation connect the

AGND pin to the negative terminal of the output capacitor

(COUT) at the load. Any drop in the high current power

ground return path will be compensated. The AGND

node carries very little current and, therefore, can be a

minimal size trace. Place a small analog bypass 0201

or 0402 ceramic capacitor as close as possible to the

LTC3307A directly on the traces leading from VIN (Pin 3)

and AGND pin. All of the signal components, such as the

FB resistor dividers and the RT resistor, should be refer-

enced to the AGND node. See the example PCB Layout

for more information.

Enable Threshold Programming

The LTC3307A has a precision threshold enable pin to

enable or disable the switching. When forced low, the

device enters a low current shutdown mode.

The rising threshold of the EN comparator is 400mV,

with 50mV of hysteresis. The EN pin can be tied to VIN

if the shutdown feature is not used. Adding a resistor

divider from VIN to EN programs the LTC3307A to regu-

late the output only when VIN is above a desired voltage

(see Figure 2). Typically, this threshold, VIN(EN), is used

in situations where the input supply is current limited, or

has a relatively high source resistance. A switching regu-

lator draws near constant power from its input source,

so source current increases as source voltage drops. This

looks like a negative resistance load to the source and can

cause the source to current limit or latch low under low

source voltage conditions. The VIN(EN) threshold prevents

the regulator from operating at source voltages where

problems may occur. This threshold can be adjusted by

setting the values R1 and R2 such that they satisfy the

following equation:

VIN EN

( ) =

+ 1

⎛

⎝

⎜

⎞

⎠

⎟ • 400mV

(10)

as shown in Figure 2:

Figure 2.

VIN

3307A F02

BUCK

SWITCHING

REGULATOR

EN Divider

The LTC3307A will remain off until VIN is above VIN(EN).

The buck regulator will remain enabled until VIN falls to

0.875 • VIN(EN) and EN is 350mV.

Alternatively, a resistor divider from an output of an

upstream regulator to the EN pin of the LTC3307A pro-

vides event-based power-up sequencing, enabling the

LTC3307A when the output of the upstream regulator

reaches a predetermined level (e.g. 90% of the regu-

lated output). Replace VIN(EN) in Equation 10 with that

predetermined level.

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