LTC3370 datasheet(16/20 Pages) AD

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LTC3302

Rev. 0

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APPLICATIONS INFORMATION

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

ground return path is compensated. The AGND node car-

ries 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 LTC3302

directly on the traces leading from AVIN (Pin 12) and

AGND (Pin 11). All of the signal components, such as

the FB resistor dividers, should be referenced to the AGND

node. See the recommended PCB Layout (Figure 3) for

more information.

Enable Threshold Programming

The LTC3302 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 LTC3302 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) thresh-

old prevents the regulator from operating at source volt-

ages where problems may occur. This threshold can be

adjusted by setting the values R1 and R2 such that they

satisfy Equation 9.

VIN EN

( ) =

+ 1

⎛

⎝⎜

⎞

⎠⎟

• 400mV

(9)

as shown in Figure 2:

Figure 2.

VIN

3302 F02

LTC3302

EN Divider

The LTC3302 remains off until VIN is above VIN(EN).

The buck regulator remains 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 LTC3302 pro-

vides event-based power-up sequencing, enabling the

LTC3302 when the output of the upstream regulator

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

lated output). Replace VIN(EN) in Equation 9 with that

predetermined level.

PCB Layout Considerations

The LTC3302 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 recommendations below to ensure proper operation.

See Figure 3 for a recommended PCB layout.

1. The VIN input supply pins (Pins 2, 3) should have local

de-coupling capacitors to the PGND pins (Pins 6, 7).

These capacitors provide the AC current to the inter-

nal power MOSFETs and their drivers. Large, switched

currents flow in these capacitors and it is important to

minimize inductance from these capacitors and their

PCB traces to the VIN and PGND pins.

• Choose a small case size, such as 0603, and place

close to the pins on the top side of the board.

• Tofurtherreducede-couplinginductance,asmaller

0201 capacitor can be placed in parallel, as close

as possible to the VIN and PGND pins.

• See CIN1 and CIN2 placement in Figure 3, the rec-

ommended PCB layout.

2. Place the regulator inductor on the same side of the

board as the LTC3302, minimizing parasitic induc-

tance from the VOUT side of the inductor and the COUT

capacitors. The power trace connecting SW to the

inductor should be wide and on PCB metal layer 2,

with as many vias as allowed to minimize added para-

sitic inductance.

3. Solder the PGND pins (Pins 6, 7) directly to a ground

plane on the top layer. Connect the top layer ground

plane to ground plane(s) on lower levels with many

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