Datasheet

LTC3617
13
3617fa
applications inForMation
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current until the feedback loop
raises the switch current enough to support the load. The
time required for the feedback loop to respond is dependent
on the compensation components and the output capaci-
tor size. Typically, 3 to 4 switching cycles are required to
respond to a load step, but only in the first cycle does the
output drop linearly. The output droop, V
DROOP
, is usually
about 2 to 4 times the linear drop of the first cycle; however,
this behavior can vary depending on the compensation
component values. Thus, a good place to start is with the
output capacitor size of approximately:
C
OUT
≈
3.5 • ∆I
OUT
f
SW
• V
DROOP
This is only an approximation; more capacitance may
be needed 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.
Output Voltage Programming
In most applications, V
OUT
is connected directly to V
FB
.
The output voltage will be equal to one-half of the voltage
on the VDDQIN pin for this case.
V
OUT
=
VDDQIN
2
If a different output relationship is desired, an external
resistor divider from V
OUT
to V
FB
can be used. The output
voltage will then be set according to the following equation:
V
OUT
=
VDDQIN
2
• 1+
R2
R1






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
OUT
shifts by an amount equal
to ∆I
LOAD
• ESR, where ESR is the effective series resis-
tance of C
OUT
. ∆I
LOAD
also begins to charge or discharge
C
OUT
, generating the feedback error signal that forces the
regulator to adapt to the current change and return V
OUT
to
its steady-state value. During this recovery time V
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
C
and C
C
) shown in Fig-
ure 1 provide adequate compensation as a starting point
for most applications. The values can be modified slightly
to optimize transient response once the final 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
C
and the bandwidth of the loop
will be increased by decreasing C
C
. If R
C
is increased by
the same factor that C
C
is decreased, the zero frequency
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
C1
, (Figure 1) is not needed for loop stability, but it
helps filter out any high frequency noise that may couple
onto that node.
LTC3617
V
FB
V
OUT
SGND
R1
3617 F03
R2
Figure 3. Setting the Output Voltage