ltc3612 Linear Technology Corporation, ltc3612 Datasheet - Page 17

• Manufacturer Part Number: ltc3612
• Description: 3a, 4mhz Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
When the ITH voltage falls below the internal (or external)
clamp voltage, the sleep state is enabled.
As the output load current drops, the peak inductor current
decreases to keep the output voltage in regulation. When
the output load current demands a peak inductor current
that is less than I
inductor current to remain equal to I
further reductions in the load current.
Since the average inductor current is greater than the output
load current, the voltage on the ITH pin will decrease. When
the ITH voltage drops, sleep mode is enabled in which
both power switches are shut off along with most of the
circuitry to minimize power consumption. All circuitry is
turned back on and the power switches resume opera-
tion when the output voltage drops out of regulation. The
value for I
output voltage ripple. As the value of I
sleep period between pulses and the output voltage ripple
increase. Note that for very high V
the power good comparator may trip, since the output
ripple may get bigger than the power good window.
Pulse-skipping mode, which is a compromise between low
output voltage ripple and effi ciency, can be implemented
by connecting MODE to SV
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 operating
discontinuously. During very light output loads, pulse
skipping allows only a few switching cycles to skip while
maintaining the output voltage in regulation.
Internal and External Compensation
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in DC load current.
When a load step occurs, V
to ΔI
of C
generating the feedback error signal that forces the regula-
tor to adapt to the current change and return V
steady-state value. During this recovery time V
OUT
LOAD(ESR)
. ΔI
BURST
LOAD
, where ESR is the effective series resistance
also begins to charge or discharge C
BURST
is determined by the desired amount of
, the burst clamp will force the peak
OUT
IN
. This sets I
shifts by an amount equal
BURST
BURST
BURST
voltage settings,
BURST
increases, the
regardless of
OUT
to 0A. In
OUT
to its
OUT
can
,
be monitored for excessive overshoot or ringing, which
would indicate a stability problem. The availability of the
ITH pin allows the transient response to be optimized over
a wide range of output capacitance.
The ITH external components (R
ure 1 provide adequate compensation as a starting point
for most applications. The values can be modifi ed slightly
to optimize transient response once the fi nal PCB layout
is done and the particular output capacitor type and value
have been determined. The output capacitors need to be
selected because the various types and values determine
the loop gain and phase. The gain of the loop will be in-
creased by increasing R
will be increased by decreasing C
the same factor that C
will be kept the same, thereby keeping the phase shift the
same in the most critical frequency range of the feedback
loop. The output voltage settling behavior is related to the
stability of the closed-loop system. The external capaci-
tor, C
helps fi lter out any high frequency noise that may couple
onto that node.
A second, more severe transient is caused by switching
in loads with large (>1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. More output
capacitance may be required depending on the duty cycle
and load step requirements.
AVP Mode
Fast load transient response, limited board space and low
cost are typical requirements of microprocessor power
supplies. A microprocessor has typical full load step with
very fast slew rate. The voltage at the microprocessor must
be held to about ±0.1V of nominal in spite of these load
current steps. Since the control loop cannot respond this
fast, the output capacitors must supply the load current
until the control loop can respond.
C1
OUT
, (Figure 1) is not needed for loop stability, but it
, causing a rapid drop in V
C
is decreased, the zero frequency
C
and the bandwidth of the loop
C
C
and C
. If R
OUT
. No regulator can
LTC3612
C
C
) shown in Fig-
is increased by
17
3612f