Detailed description, Pin description, Typical operating characteristics (continued) – Rainbow Electronics MAX1697 User Manual
Page 7: Efficiency considerations
Detailed Description
The MAX1697 capacitive charge pumps invert the volt-
age applied to their input. For highest performance, 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
IN
volts. This connects C1 in par-
allel with the reservoir capacitor C2. If the voltage across
C2 is smaller than the voltage across C1, charge flows
from C1 to C2 until the voltage across C2 reaches
-V
IN
. The actual voltage at the output is more positive
than -V
IN
, since switches S1–S4 have resistance and the
load drains charge from C2.
Efficiency Considerations
The efficiency of the MAX1697 is dominated by its qui-
escent supply current (I
Q
) at low output current and by
its output impedance (R
OUT
) at higher output current; it
is given by:
I
I
I
1
I
x R
V
OUT
OUT
Q
OUT
OUT
IN
η ≅
+
−
MAX1697
60mA, SOT23 Inverting Charge Pump
with Shutdown
_______________________________________________________________________________________
7
0
100
50
200
150
300
250
350
450
400
500
0
2
3
4
1
5
6
7
9
8
10
MAX1697T
OUTPUT RIPPLE vs. CAPACITANCE
MAX1697 TOC27
CAPACITANCE (
µF)
OUTPUT RIPPLE (mV)
V
IN
= +1.9V, V
OUT
= -1.5V
V
IN
= +3.15V, V
OUT
= -2.5V
V
IN
= +4.75V, V
OUT
= -4.0V
0
100
50
200
150
300
250
350
450
400
500
0
2
3
4
1
5
6
7
9
8
10
MAX1697U
OUTPUT RIPPLE vs. CAPACITANCE
MAX1697 TOC28
CAPACITANCE (
µF)
OUTPUT RIPPLE (mV)
V
IN
= +1.9V, V
OUT
= -1.5V
V
IN
= +3.15V, V
OUT
= -2.5V
V
IN
= +4.75V, V
OUT
= -4.0V
Pin Description
6
Positive Terminal of the Flying
Capacitor
1
Inverting Charge-Pump Output
2
Power-Supply Voltage Input. Input
range is 1.5V to 5.5V.
3
Negative Terminal of the Flying
Capacitor
4
Ground
5
Shutdown Input. Drive this pin high
for normal operation; drive it low for
shutdown mode. OUT is actively
pulled to ground during shutdown.
PIN
FUNCTION
NAME
C1+
OUT
IN
C1-
GND
SHDN
Typical Operating Characteristics (continued)
(Circuit of Figure 1, capacitors from Table 2, V
IN
= +5V, SHDN = IN, T
A
= +25°C, unless otherwise noted.)
TE: (
C1
C2
2
1
5
ON
OFF
3
R
L
6
4
C3
C1+
C1-
IN
SHDN
OUT
GND
INPUT
1.5V to 5.5V
NEGATIVE
OUTPUT
-1
✕
V
IN
MAX1697
Figure 1. Typical Application Circuit