LTC3414IFE Datasheet, PDF (10/16 Pages) Linear Technology – 4A, 4MHz, Monolithic Synchronous Step-Down Regulator

LTC3414
APPLICATIO S I FOR ATIO
Burst Clamp Programming
If the voltage on the SYNC/MODE pin is less than V_IN by 1V,
Burst Mode operation is enabled. During Burst Mode
Operation, the voltage on the SYNC/MODE pin determines
the burst clamp level, which sets the minimum peak
inductor current, I_BURST, for each switching cycle accord-
ing to the following equation:
( ) ^IBURST
=
⎛
⎝⎜
6.9A ^⎞
0.6V ⎠⎟
V_BURST
– 0.383V
V_BURST is the voltage on the SYNC/MODE pin. I_BURST can
only be programmed in the range of 0A to 7A. For values
of V_BURST greater than 1V, I_BURST is set at 7A. For values
of V_BURST less than 0.4V, I_BURST is set at 0A. As the output
load current drops, the peak inductor currents decrease to
keep the output voltage in regulation. When the output
load current demands a peak inductor current that is less
than I_BURST, the burst clamp will force the peak inductor
current to remain equal to I_BURST regardless of further
reductions in the load current. Since the average inductor
current is greater than the output load current, the voltage
on the I_TH pin will decrease. When the I_TH voltage drops
to 150mV, sleep mode is enabled in which both power
MOSFETs are shut off along with most of the circuitry to
minimize power consumption. All circuitry is turned back
on and the power MOSFETs begin switching again when
the output voltage drops out of regulation. The value for
I_BURST is determined by the desired amount of output
voltage ripple. As the value of I_BURST increases, the sleep
period between pulses and the output voltage ripple in-
crease. The burst clamp voltage, V_BURST, can be set by a
resistor divider from the V_FB pin to the SGND pin as shown
in Figure 1.
Pulse skipping, which is a compromise between low
output voltage ripple and efficiency, can be implemented
by connecting pin SYNC/MODEto ground. This sets I_BURST
to 0A. In this condition, the peak inductor current is limited
by the minimum on-time of the current comparator. The
lowest output voltage ripple is achieved while still operat-
ing discontinuously. During very light output loads, pulse
skipping allows only a few switching cycles to be skipped
while maintaining the output voltage in regulation.
10
Frequency Synchronization
The LTC3414’s internal oscillator can be synchronized to
an external clock signal. During synchronization, the top
MOSFET turn-on is locked to the falling edge of the
external frequency source. The synchronization frequency
range is 300kHz to 4MHz. Synchronization only occurs if
the external frequency is greater than the frequency set
by the external resistor. Because slope compensation is
generated by the oscillator’s RC circuit, the external
frequency should be set 25% higher than the frequency
set by the external resistor to ensure that adequate slope
compensation is present.
Soft-Start
The RUN/SS pin provides a means to shut down the
LTC3414 as well as a timer for soft-start. Pulling the
RUN/SS pin below 0.5V places the LTC3414 in a low
quiescent current shutdown state (I_Q < 1μA).
The LTC3414 contains an internal soft-start clamp that
gradually raises the clamp on I_TH after the RUN/SS pin is
pulled above 2V. The full current range becomes available
on I_TH after 1024 switching cycles. If a longer soft-start
period is desired, the clamp on I_TH can be set externally
with a resistor and capacitor on the RUN/SS pin as shown
in Figure 1. The soft-start duration can be calculated by
using the following formula:
t_SS
=
R_SS
C _SS
⎛
ln _{⎝⎜ VIN}
V_IN ^⎞
– 1.8V⎠⎟
(SECONDS)
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses: V_IN quiescent current and I²R losses.
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