LTC3448_15 Datasheet, PDF (7/20 Pages) Linear Technology – 1.5MHz/2.25MHz, 600mA Synchronous Step-Down Regulator with LDO Mode

W
FU CTIO AL DIAGRA
SYNC
7
LTC3448
MODE
3
FREQ
6
V_IN
OSC
V_OUT
2
LDO
DRIVE
SLOPE
COMP
LDO CONTROL
LOGIC
4 V_IN
V_FB
1
V_IN
RUN
8
0.6V REF
0.6V ⁺
– ^EA
0.6V + ∆OVL
–
OVDET
+
OSC
SQ
RQ
RS LATCH
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
I_COMP
ANTI-
SHOOT-
THRU
5Ω
5 SW
SHUTDOWN
Figure 1
I_RCMP
9 GND
3448 F01
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC3448 uses a constant frequency, current mode,
step-down architecture. Both the main (P-channel MOS-
FET) 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 comparator,
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_INT relative to the 0.6V 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 re-
verse, as indicated by the current reversal comparator
I_RCMP, or the beginning of the next clock cycle. The
comparator OVDET guards against transient overshoots
5.8% by turning off the main switch and keeping it off until
the fault is removed.
Pulse Skipping Mode Operation
At light loads, the inductor current may reach zero or
reverse on each pulse. The bottom MOSFET is turned off
by the current reversal comparator, I_RCMP, and the switch
voltage will ring. This is discontinuous mode operation,
and is normal behavior for the switching regulator. At very
light loads, the LTC3448 will automatically skip pulses to
maintain output regulation.
Low Ripple LDO Mode Operation
At load currents below I_LDO(ON), and when enabled, the
LTC3448 will switch into very low ripple, linear regulating
operation (LRO). In this mode, the current is sourced from
3448f
7