LTC3406-1.2 Datasheet, PDF (6/12 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406-1.2
U
OPERATIO (Refer to Functional Diagram)
V_IN
2.7V
TO 5.5V
C_IN**
4.7µF
CER
4
V_IN
3 ^2.2µH*
SW
LTC3406-1.2
1
RUN
5
V_OUT
GND
2
C_OUT^†
10µF
CER
V_OUT
1.2V
600mA
340612 F01
*MURATA LQH3C2R2M24
**TAIYO YUDEN JMK212BJ475MG
†_{TAIYO YUDEN JMK316BJ106ML}
Figure 1. Typical Application
Main Control Loop
The LTC3406-1.2 uses a constant frequency, current
mode step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
are internal. During normal operation, the internal top
power MOSFET is turned on each cycle when the oscillator
sets the RS latch, and turned off when the current com-
parator, I_COMP, resets the RS latch. The peak inductor
current at which I_COMP resets the RS latch, is controlled by
the output of error amplifier EA. When the load current
increases, it causes a slight decrease in the feedback
voltage, FB, relative to the 0.8V reference, which in turn
causes the EA amplifier’s output voltage to increase until
the average inductor current matches the new load cur-
rent. While the top MOSFET is off, the bottom MOSFET is
turned on until either the inductor current starts to reverse,
as indicated by the current reversal comparator I_RCMP, or
the beginning of the next clock cycle. The comparator
OVDET guards against transient overshoots >6.25% by
turning the main switch off and keeping it off until the fault
is removed.
Burst Mode Operation
The LTC3406-1.2 is capable of Burst Mode operation in
which the internal power MOSFETs operate intermittently
based on load demand.
In Burst Mode operation, the peak current of the inductor
is set to approximately 200mA regardless of the output
load. Each burst event can last from a few cycles at light
loads to almost continuously cycling with short sleep
intervals at moderate loads. In between these burst events,
the power MOSFETs and any unneeded circuitry are turned
off, reducing the quiescent current to 20µA. In this sleep
state, the load current is being supplied solely from the
output capacitor. As the output voltage droops, the EA
amplifier’s output rises above the sleep threshold signal-
ing the BURST comparator to trip and turn the top MOSFET
on. This process repeats at a rate that is dependent on the
load demand.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator is reduced to about 210kHz, 1/7 the nominal
frequency. This frequency foldback ensures that the in-
ductor current has more time to decay, thereby preventing
runaway. The oscillator’s frequency will progressively
increase to 1.5MHz when V_OUT rises above 0V.
340612f
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