LT8614 datasheet(16/24 Pages) AD

16 / 24 page

LTC3315A

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

creating 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 the 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.

Whenusingtheloadtransientresponsemethodtostabilize

the control loop, apply an output current pulse going from

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 149).

Using the Precision Enable Threshold

The LTC3315A has precision threshold enable pins for

each buck regulator to enable or disable each buck. When

both are forced low, the device enters into a low current

shutdown mode.

The rising threshold of both EN comparators is 400mV,

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

if the shutdown feature is not used. Adding a resistor

divider from VIN to an EN pin to ground programs the

LTC3315A to regulate that output only when VIN is above

a desired voltage.

Typically, this threshold, VIN(EN), is used in situations

where the input supply is current limited, or has a rela-

tively high source resistance. A switching regulator draws

near constant power from its input source, so source cur-

rent 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 prob-

lems may occur. Referring to Figure 2, this threshold can

Figure 2. EN Divider

VIN

3315ab F02

BUCK

SWITCHING

REGULATOR

be adjusted by setting the values of R3 and R4 such that

they satisfy the following equation:

VIN(EN) = 400mV • 1+

⎛

⎝

⎜

⎞

⎠

⎟

(9)

The buck regulator 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 the output of one buck

to the EN pin of the second buck to ground provides event

based power-up sequencing as the first buck reaching

regulation enables the second buck. Replace VIN(EN) in

Equation 9 with the desired output voltage of the first buck

(e.g. 90% of the regulated value) at which the second

buck is enabled.

PCB Considerations

The LTC3315A is a high performance IC designed for high

efficiency and fast transient response. For optimal results

carefully consider the layout of the PCB board and follow

the below list to ensure proper operation. See Figure 3 for

a recommended PCB layout.

1. Connect the exposed pad of the package (Pin 13)

directly to a large, unbroken ground plane under the

application circuit on the layer closest to the sur-

face layer to minimize thermal and electrical imped-

ance. Additionally, short the exposed pad to ground

pins 8 and 9 on the top layer. See the Analog Devices

Application Note, Application Notes for Thermally

Enhanced Leaded Plastic Packages for the proper size

and layout of the thermal vias and solder stencils.

2. Both of the input supply pins should have local decou-

pling capacitors with their grounded pins connecting

on the top layer to the ground plane around pin 8,

LT8614 Datasheet(PDF) 16 Page - Analog Devices

LT8614 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL