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LTC3615

3615f

(7a) Slew Rate of Rising Edge at SW1/2 vs SRLIM Resistor

The LTC3615 allows the user to control the slew rate of

the switching node SW by using the SRLIM pin. Tying this

pin to ground selects the fastest slew rate. The slowest

slew rate is selected when the pin is open. Connecting a

resistor (between 10k to 100k) from SRLIM pin to ground

adjusts the slew rate between the maximum and minimum

values. The reduced dV/dt of the switch node results in a

significant reduction of the supply and ground ringing, as

well as lower radiated EMI. See Figure 7a and the Typical

Performance Characteristics section for examples.

Reducing the slew rate causes a trade-off between ef-

ficiency and low EMI (see Figure 7b).

Particularattentionshouldbeusedwithveryhighswitching

frequencies. Using the slowest slew rate (SRLIM open)

can reduce the minimum duty cycle capability.

Soft-Start

The RUNx pins provide a means to shut down each chan-

nel of the LTC3615. Pulling both pins below 0.3V places

the LTC3615 in a low quiescent current shutdown state

(IQ < 1µA).

After enabling the LTC3615 by bringing either one or both

RUNxpinsabovethethreshold,theenabledchannelsenter

a soft-start-up state. The type of soft-start behavior is set

by the TRACK/SSx pins. The soft-start cycle begins with

an initial discharge pulse pulling down the TRACK/SSx

pin to SGND and discharging the external capacitor CSS

(see Figure 3).

The initial discharge is adequate to discharge capacitors

up to 33nF. If a larger capacitor is required, connect the

external soft-start resistor RSS to the RUN pin to fully

discharge the capacitor.

1. Tying this pin to SVIN selects the internal soft-start

circuit. This circuit ramps the output voltage to the final

value within 1ms.

2. If a longer soft-start period is desired, it can be set ex-

ternally with a resistor and capacitor on the TRACK/SSx

pins as shown in Figure 3. The voltage applied at the

TRACK/SSx pins sets the value of the internal refer-

ence at VFB until TRACK/SSx is pulled above 0.6V. The

external soft-start duration can be calculated by using

the following equation:









•

– .

0 6

3. The TRACK/SSx pin can be used to track the output

voltage of another supply.

Regardless of either the internal or external soft-start

state, the MODE pin is ignored during start-up and the

regulator defaults to pulse-skipping mode. In addition,

the PGOODx pin is kept low, and the frequency foldback

function is disabled.

applicaTions inForMaTion

(7b) Efficiency vs SRLIM Resistor Programming

Figure 7. Slew Rate and the SRLIM Resistor

1V/DIV

2ns/DIV

3615 F07a

SRLIM =

SGND OR SVIN

VIN = 3.3V

VOUT = 1.8V

IOUT = 1A

OPEN

40.2k

100k

VIN (V)

2.25

3.06

3.88

4.69

5.50

3615 07b

VOUT = 1.8V

IOUT = 1A

FCM

GND OR SVIN

OPEN

40.2k

20k

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