MAX1697 Maxim, MAX1697 Datasheet - Page 8

• Manufacturer Part Number: MAX1697
• Description: 60mA / SOT23 Inverting Charge Pump with Shutdown
• Manufacturer: Maxim

60mA, SOT23 Inverting Charge Pump
with Shutdown
Figure 2. Ideal Voltage Inverter
8
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 4Ω to 5Ω at V
cal output impedance is more accurately determined
from the Typical Operating Characteristics .
The MAX1697 limits its input current upon start-up to
170mA (typ). This prevents low-current or higher output
impedance input supplies (such as alkaline cells) from
being overloaded when power is applied or when the
device awakes from shutdown.
The MAX1697 has a logic-controlled shutdown input.
Driving SHDN low places the device in a low-power
shutdown mode. The charge-pump switching halts,
supply current is reduced to 2nA, and OUT is actively
pulled to ground through a 3Ω resistance.
Driving SHDN high will restart the charge pump. The
switching frequency and capacitor values determine
how soon the device will reach 90% of the input voltage.
Figure 3a. Switched-Capacitor Model
SW
_______________________________________________________________________________________
is the sum of the charge pump’s internal switch
IN
R
OUT
V+
C1
S3
S1
f
OSC
f
OSC
1
C1
C1
2R
S4
S2
SW
C2
4ESR
IN
C2
= +5V). The typi-
Current Limit
C1
R
L
V
OUT
Shutdown
ESR
V
OUT
C2
= -(V
IN
)
The MAX1697 has a thermal shutdown mode for addi-
tional protection against fault conditions. When the tem-
perature of the die exceeds +150°C, the internal clock
stops, suspending the device’s operation. The
MAX1697 resumes operation when the die temperature
falls 15°C. This prevents the device from rapidly oscil-
lating around the temperature trip point.
The charge-pump output resistance is a function of the
ESR of C1 and C2. To maintain the lowest output resis-
tance, use capacitors with low ESR. (See Table 1 for a
list of recommended manufacturers.) Tables 2 and 3
suggest capacitor values for minimizing output resis-
tance or capacitor size.
Increasing the flying capacitor’s value reduces the out-
put resistance. Above a certain point, increasing C1’s
capacitance has negligible effect because the output
resistance is then dominated by internal switch resis-
tance and capacitor ESR.
Increasing the output capacitor’s value reduces the
output ripple voltage. Decreasing its ESR reduces both
output resistance and ripple. Lower capacitance values
can be used with light loads if higher output ripple can
be tolerated. Use the following equation to calculate the
peak-to-peak ripple:
If necessary, bypass the incoming supply to reduce its
AC impedance and the impact of the MAX1697’s switch-
ing noise. A bypass capacitor with a value equal to that
of C1 is recommended.
Figure 3b. Equivalent Circuit
V
RIPPLE
V+
R
EQUIV
Applications Information
R
=
EQUIV
=
2(f
f
OSC
OSC
I
1
OUT
·
C1
)C2
Input Bypass Capacitor (C3)
C2
Capacitor Selection
2 I
Thermal Shutdown
Output Capacitor (C2)
Flying Capacitor (C1)
OUT
ESR
R
L
V
C2
OUT