Rainbow Electronics MAX889 User Manual
Page 7
MAX889
High-Frequency, Regulated,
200mA, Inverting Charge Pump
_______________________________________________________________________________________
7
Adjust the output voltage to a negative voltage from
-2.5V to -V
IN
with external resistors R1 and R2 as
shown in Figures 1 and 4. FB servos to GND. Choose
R1 to be 100k
Ω or less. Calculate R2 for the desired
output voltage:
V
OUT
= -V
REF
(R2 / R1)
R2 = R1 (V
OUT
/ -V
REF
)
where V
REF
can be either V
IN
or a positive reference
source.
Typically, choose a voltage-divider current of at least
30µA to minimize the effect of FB input current and
capacitance:
R1
≤ V
REF
/ 30µA
R2 < -V
OUT
/ 30µA
Capacitor Selection
The appropriate capacitors used with the MAX889
depend on the switching frequency. Table 1 provides
suggested values for C
IN
, C
FLY
, and C
OUT
.
Surface-mount ceramic capacitors are preferred for
C
IN
, C
OUT
, and C
FLY
due to their small size, low cost,
and low ESR. To ensure proper operation over the
entire temperature range, choose ceramic capacitors
with X7R (or equivalent) low-temperature-coefficient
(tempco) dielectrics. See Table 2 for a list of suggested
capacitor suppliers.
The output capacitor stores the charge transferred from
the flying capacitor and services the load between
oscillator cycles. A good general rule is to make the
output capacitance at least five-times greater than the
flying capacitor.
Output voltage ripple is largely dependent on C
OUT
.
Choosing a low-ESR capacitor of sufficient value is impor-
tant in minimizing the peak-to-peak output voltage ripple,
which is approximated by the following equation:
where C
OUT
is the output capacitor value, ESR
COUT
is
the output capacitor’s ESR, and f
OSC
is the MAX889
switching frequency. Ceramic capacitors have the lowest
ESR and are recommended for C
OUT
. Where larger
capacitance at low cost is desired, a low-ESR tantalum
capacitor may be used for C
OUT
. See Table 2 for a list of
suggested capacitor suppliers.
To ensure stability over the entire operating temperature
range, choose a low-ESR output capacitor using the fol-
lowing equation:
where C
OUT
is the output capacitor value, and f
MIN
is the
minimum oscillator frequency in the Electrical
Characteristics table.
To ensure stability for regulated output mode, suitable
output capacitor ESR should be determined by the follow-
ing equation:
Power Dissipation
The power dissipated in the MAX889 depends on the
input voltage, output voltage, and output current. Device
power dissipation is accurately described by:
P
DISS
= I
OUT
(V
IN
- (-V
OUT
)) + (I
Q
✕
V
IN
)
where I
Q
is the device quiescent current. P
DISS
must be
less than the package dissipation rating (see Absolute
Maximum Ratings). Pay particular attention to power dis-
sipation limits when generating small negative voltages
from large positive input voltages.
Layout Considerations
The MAX889’s high oscillator frequencies demand
good layout techniques that ensure stability and help
maintain the output voltage under heavy loads. Take
the following steps to ensure optimum layout:
1) Mount all components as close together as possible.
2) Place the feedback resistors R1 and R2 close to the
FB pin, and minimize the PC trace length at the FB
circuit node.
3) Keep traces short to minimize parasitic inductance
and capacitance.
4) Use a ground plane with C
IN
and C
OUT
placed in a
star ground configuration (see the MAX889SEVKIT
layout).
R
19.2 x 10
I
R2
R1
ESR
-3
OUT
≤
+
1
C
15.5
f
R1
R1 + R2
I
OUT
MIN
OUT
≥
V
=
I
2 x f
C
2 x I
ESR
RIPPLE
OUT
OSC
OUT
OUT
COUT
+