Datasheet

EVAL-ADP2105
Rev. 0 | Page 4 of 12
MODIFYING THE EVALUATION BOARD
The ADP2105 evaluation board is supplied fully assembled and
tested for proper operation. It comes in two versions: the
ADP2105-1.8-EVAL with fixed output voltage of 1.8 V and the
ADP2105-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 ADP2105-EVAL output regulation voltage can be changed
by altering its external components. The ADP2105-1.8-EVAL
output regulation voltage is fixed at 1.8 V and cannot be
changed.
The ADP2105-EVAL output regulation voltage is set by a resistive
voltage divider consisting of Resistors R4 and R5. Resistor R4
corresponds to the R
TOP
resistor in the ADP2105 data sheet, and
Resistor R5 corresponds to the R
BOT
resistor in the ADP2105
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
= 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
, by
⎥
⎦
⎤
⎢
⎣
⎡
−
=
FB
FBOUT
BOTTOP
V
VV
RR
For example, to set the output regulation voltage of the
ADP2105-EVAL to 2.0 V, calculate the value of Resistor R4 and
Resistor R5 as shown below.
Ω=== k40
μ20
V8.0
AI
V
R5
STRING
FB
Ω=
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
−
×Ω=
⎥
⎦
⎤
⎢
⎣
⎡
−
×= k60
V8.0
V8.0V2
k40
FB
FB
OUT
V
VV
R5R4
Note that when the output voltage of ADP2105-EVAL is
changed, the output capacitors (C3 and C4), inductor (L1), and
compensation components (R1 and C6) are recalculated and
changed according to the Application Information section in
the
ADP2105 data sheet to ensure stable operation.
Changing the Load Transient Response
The ADP2105 evaluation board load transient response can be
altered by changing the output capacitors (C3 and C4) and
compensation components (R1 and C6) as explained in the
Output Capacitor Selection and Loop Compensation sections of
the
ADP2105 data sheet. By default, the load transient response
of the ADP2105 evaluation board is set to 5% of the output
voltage for a 1 A load transient.
Consider an example where the load transient response of the
ADP2105-1.8-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
ADP2105 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
ADP2105 data sheet, let C3 and C4 be
two 10 μF X5R MLCC capacitors (GRM21BR61A106KE19L).
Finally, calculate the compensation resistor and compensation
capacitor as shown below:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
REF
OUTOUT
CSm
CROSS
COMP
V
VC
GG
F
R
)π2(
8.0
Ω=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
×
= k135
V8.0
V8.1μF14
V/A875.1V/μA50
kHz80)π2(
8.0
pF60
kΩ135kHz80π
2
π
2
=
××
==
COMPCROSS
COMP
RF
C
Therefore, the compensation resistor is 135 kΩ and the
compensation capacitor is 68 pF.