LTC3615 Datasheet, PDF (21/32 Pages) Linear Technology – Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter

LTC3615
Applications Information
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).
Particular attention should be used with very high switching
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
(I_Q < 1µA).
After enabling the LTC3615 by bringing either one or both
RUNx pins above the threshold, the enabled channels enter
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 C_SS
(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 R_SS to the RUN pin to fully
discharge the capacitor.
1. Tying this pin to SV_IN 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 V_FB until TRACK/SSx is pulled above 0.6V. The
external soft-start duration can be calculated by using
the following equation:
t_SS
=
R_SS
•
C_SS
•
In


SV_IN
SV_IN – 0.6V


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.
V_IN = 3.3V
V_OUT = 1.8V
I_OUT = 1A
SRLIM =
SGND OR SV_IN
1V/DIV
40.2k
100k
OPEN
2ns/DIV
3615 F07a
(7a) Slew Rate of Rising Edge at SW1/2 vs SRLIM Resistor
92
V_OUT = 1.8V
91 I_OUT = 1A
FCM
90
89
88 ^40.2k
20k
87
GND OR SV_IN
OPEN
86
85
84
83
82
2.25
3.06
3.88
4.69
5.50
V_IN (V)
3615 07b
(7b) Efficiency vs SRLIM Resistor Programming
Figure 7. Slew Rate and the SRLIM Resistor
3615f
21