MAX871EUK-T Maxim Integrated Products, MAX871EUK-T Datasheet - Page 5

• Manufacturer Part Number: MAX871EUK-T
• Description: Charge Pumps Switched-Capacitor Voltage Inverter
• Manufacturer: Maxim Integrated Products

Specifications of MAX871EUK-T

• Function: Inverting
• Output Voltage: - 1.4 V to - 5.5 V
• Output Current: 25 mA
• Maximum Operating Temperature: + 85 C
• Package / Case: SOT-23-5
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT

The MAX870/MAX871 capacitive charge pumps invert
the voltage applied to their input. For highest perfor-
mance, use low equivalent series resistance (ESR)
capacitors (e.g., ceramic).
During the first half-cycle, switches S2 and S4 open,
switches S1 and S3 close, and capacitor C1 charges to
the voltage at IN (Figure 2). During the second half-
cycle, S1 and S3 open, S2 and S4 close, and C1 is level
shifted downward by V
allel with the reservoir capacitor C2. If the voltage across
C2 is smaller than the voltage across C1, then charge
flows from C1 to C2 until the voltage across C2 reaches
-V
than -V
load drains charge from C2.
The MAX870/MAX871 are not voltage regulators: the
charge pump’s output source resistance is approxi-
mately 20Ω at room temperature (with V
V
droop toward GND as load current increases. The
droop of the negative supply (V
rent draw from OUT (I
er’s source resistance (RS-):
The negative output voltage will be:
The efficiency of the MAX870/MAX871 is dominated by
its quiescent supply current (I
and by its output impedance (R
current; it is given by:
Figure 3a. Switched-Capacitor Model
_______________Detailed Description
OUT
IN
. The actual voltage at the output is more positive
approaches -5V when lightly loaded. V
IN
V+
, since switches S1–S4 have resistance and the
η ≅
C1
I
OUT
V
V
OUT
I
DROOP-
OUT
f
+
_______________________________________________________________________________________
I
= -(V
Q
OUT
Efficiency Considerations
IN



= I
1
Switched-Capacitor Voltage Inverters
IN
volts. This connects C1 in par-
−
OUT
) times the negative convert-
– V
Charge-Pump Output
I
C2
OUT
DROOP-
x RS-
Q
DROOP-
) at low output current
OUT
V
×
IN
R
) at higher output
)
OUT
) equals the cur-
R
IN
L
V
OUT
= +5V), and



OUT
will
where the output impedance is roughly approximated
by:
The first term is the effective resistance of an ideal
switched-capacitor circuit (Figures 3a and 3b), and
R
resistances (typically 8Ω to 9Ω at V
output impedance is more accurately determined from
the Typical Operating Characteristics.
To maintain the lowest output resistance, use capacitors
with low ESR (Table 1). The charge-pump output resis-
tance is a function of C1’s and C2’s ESR. Therefore,
minimizing the charge-pump capacitor’s ESR minimizes
the total output resistance.
Figure 2. Ideal Voltage Inverter
Figure 3b. Equivalent Circuit
__________Applications Information
SW
R
OUT
is the sum of the charge pump’s internal switch
IN
≅
V+
(
f
OSC
R
S3
S1
EQUIV
R
1
)
EQUIV
=
x C
C1
f × C1
1
1
+
2
R
S4
S2
SW
C2
Capacitor Selection
+
4
IN
ESR
C2
= +5V). The typical
R
C
L
1
V
+
OUT
ESR
V
OUT
= -(V
C
2
IN
)
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