LTC3624_15 Datasheet, PDF (12/20 Pages) Linear Technology – 17V, 2A Synchronous Step-Down Regulator with 3.5A Quiescent Current

LTC3624/LTC3624-2
Applications Information
usually about three times the linear drop of the first cycle.
Thus, a good place to start with the output capacitor value
is approximately:
C_OUT
=
3
f
ΔI_OUT
• V_DROOP
More capacitance may be required depending on the duty
cycle and load-step requirements. In most applications,
the input capacitor is merely required to supply high
frequency bypassing, since the impedance to the supply
is very low. A 10μF ceramic capacitor is usually enough
for these conditions. Place this input capacitor as close
to the V_IN pin as possible.
Output Power Good
When the LTC3624/LTC3624-2’s output voltage is within
the ±7.5% window of the regulation point, the output
voltage is good and the PGOOD pin is pulled high with
an external resistor. Otherwise, an internal open-drain
pull-down device (280Ω) will pull the PGOOD pin low. To
prevent unwanted PGOOD glitches during transients or
dynamic V_OUT changes, the LTC3624/LTC3624-2’s PGOOD
falling edge includes a blanking delay of approximately 32
switching cycles.
Frequency Sync Capability
The LTC3624/LTC3624-2 has the capability to sync to a
±40% range of the internal programmed frequency. It
takes 2 to 3 cycles of external clock to engage the sync
mode, and roughly 2µs of no clocks for the part to real-
ize that the sync signal is gone. Once engaged in sync,
the LTC3624/LTC3624-2 immediately runs at the external
clock frequency.
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple current:
∆I_L
=
V_OUT
f •L
_1–

V_{OUT }
V_IN(MAX) 
Lower ripple current reduces power losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a trade-off between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). To guarantee that ripple
current does not exceed a specified maximum, the induc-
tance should be chosen according to:
L
=
f
•
V_OUT
∆I_L(MAX)
_1–

V_OUT
V_IN(MAX)


Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for a fixed inductor value, but is very dependent on the
inductance selected. As the inductance or frequency in-
creases, core losses decrease. Unfortunately, increased
inductance requires more turns of wire and therefore
copper losses will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price versus size requirements
and any radiated field/EMI requirements. New designs for
surface mount inductors are available from Toko, Vishay,
Coilcraft, NEC/Tokin, Cooper, TDK and Würth Electronik.
Refer to Table 1 for more details.
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