Datasheet

EVAL-ADP2107
Rev. 0 | Page 4 of 12
MODIFYING THE EVALUATION BOARD
The ADP2107 evaluation board is supplied fully assembled and
tested for proper operation. It comes in two versions: the
ADP2107-1.8-EVAL with fixed output voltage of 1.8 V and the
ADP2107-EVAL with adjustable output voltage initially set to 2.5 V.
The two most common modifications that can be done to the
evaluation boards are changing the output voltage and changing
the load transient response.
CHANGING THE OUTPUT VOLTAGE
The ADP2107-EVAL output regulation voltage can be changed
by altering its external components. The ADP2107-1.8-EVAL
output regulation voltage is fixed at 1.8 V and cannot be changed.
The ADP2107-EVAL output regulation voltage is set by a
resistive voltage divider consisting of Resistor R4 and Resistor R5.
Resistor R4 corresponds to the R
TOP
resistor in the ADP2107
data sheet, and Resistor R5 corresponds to the R
BOT
resistor in
the ADP2107 data sheet. The output regulation voltage is
determined by the equation
⎥
⎦
⎤
⎢
⎣
⎡
+
×=
BOT
BOTTOP
OUT
R
RR
V V8.0
where:
R
TOP
is the value of the top resistor of the voltage divider (R4).
R
BOT
is the value of the bottom resistor of the voltage divider (R5).
V
OUT
is the output regulation voltage in volts.
To set the output regulation voltage to the desired value, first
determine the value of the bottom resistor, R
BOT
, by
STRING
FB
BOT
I
V
R =
where:
V
FB
is 0.8 V, the internal reference.
I
STRING
is the resistor divider string current (20 μA nominally).
Once R
BOT
is determined, calculate the value of the top resistor,
R
TOP
, from
⎥
⎦
⎤
⎢
⎣
⎡
−
=
FB
FBOUT
BOTTOP
V
VV
RR
For example, to set the output regulation voltage of ADP2107-
EVAL to 2.0 V, calculate the value of Resistor R4 and Resistor
R5 as shown below.
Ω=== k40
μA20
V8.0
STRING
FB
I
V
R5
Ω=
⎥
⎦
⎤
⎢
⎣
⎡
−
×Ω=
⎥
⎦
⎤
⎢
⎣
⎡
−
×= k60
V8.0
V8.0V2
k40
FB
FBOUT
V
VV
R5R4
Note that when the output voltage of the ADP2107-EVAL is
changed, the output capacitors (C3 and C4), the inductor (L1),
and the compensation components (R1 and C6) must be
recalculated and changed according to the Application
Information section in the ADP2107 data sheet to ensure stable
operation.
CHANGING THE LOAD TRANSIENT RESPONSE
The ADP2107 evaluation board load transient response can be
altered by changing the output capacitors (C3 and C4) and the
compensation components (R1 and C6) as explained in the
Output Capacitor section and Loop Compensation section of
the ADP2107 data sheet. By default, the load transient response
of both ADP2107 evaluation boards is set to 5% of the output
voltage for a 1 A load transient.
Consider an example where the load transient response of
ADP2107-1.8-EVAL is changed to 10% of the output voltage for
a 1 A load transient.
First, choose the output capacitors (C3 and C4) based on the load
transient response requirements. The desired load transient
response is 10% overshoot for a 1 A load transient. For this
condition, the % Overshoot for a 1 A Load Transient Response vs.
Output Capacitor × Output Voltage figure in the Output
Capacitor Selection section of the ADP2107 data sheet gives
Output Capacitor × Output Voltage = 25 μC
μF14
V8.1
μC25
≈=⇒ CapacitorOutput
Next, taking into account the loss of capacitance due to dc bias
as shown in the % Drop-In Capacitance vs. DC Bias for Ceramic
Capacitors figure in the Output Capacitor Selection section of
the ADP2107 data sheet, let C3 and C4 be two 10 μF X5R MLCC
capacitors (GRM21BR61A106KE19L).
Finally, calculate the compensation resistor and compensation
capacitor as follows:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
REF
OUTOUT
CSm
CROSS
COMP
V
VC
GG
F
R
)π2(
8.0
Ω=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
×
= k70
V8.0
V8.1μF14
V/A625.3V/μA50
kHz80)π2(
8.0
pF120
kΩ70kHz80π
2
π
2
=
××
==
COMPCROSS
COMP
RF
C
Therefore, choose the compensation resistor to be 70 kΩ and
the compensation capacitor to be 120 pF.