MAX1673 Maxim, MAX1673 Datasheet - Page 7

• Manufacturer Part Number: MAX1673
• Description: Regulated / 125mA-Output / Charge-Pump DC-DC Inverter
• Manufacturer: Maxim

In LIN mode (LIN/SKIP = IN), the charge pump runs con-
tinuously at 350kHz. The MAX1673 controls the charge
on C
When the output voltage falls, C
to increased gate drive. Since the device switches con-
tinuously, the regulation scheme minimizes output ripple,
the output noise contains well-defined frequency compo-
nents, and the circuit requires much smaller external
capacitors than in Skip mode for a given output ripple.*
However, LIN mode is less efficient than Skip mode due
to higher operating current (8mA typical).
In Skip mode (LIN/SKIP = GND), the device switches
only as needed to maintain regulation on FB. Switching
cycles are skipped until the voltage on FB rises above
GND. Skip mode has higher output noise than LIN
mode, but minimizes operating current.
When SHDN (a CMOS-compatible input) is driven low,
the MAX1673 enters low-power shutdown mode.
Charge-pump switching action halts and an internal 1Ω
switch pulls V
drive high for normal operation.
*See Output Ripple vs. Load Current in Typical Operating Characteristics .
The accuracy of V
voltage biasing the voltage-divider network (R1, R2).
Use a separate reference voltage if V
ed voltage or if greater accuracy is desired (Figure 4).
Adjust the output voltage from -1.5V to -V
mode or 0V to -V
R1 and R2 as shown in Figures 1 and 4. In either regu-
lating mode (LIN or Skip), FB servos to 0V. Use the
following equations to select R1 and R2 for the desired
output voltage:
where V
reference source.
Typically, choose a voltage-divider current of 50µA to
minimize the effect of FB input current:
Linear Mode (Constant-Frequency Mode)
FLY
REF
by varying the gate drive on S1 (Figure 2).
can be either V
OUT
V
Applications Information
OUT
IN
_______________________________________________________________________________________
OUT
R1 = V
R2 = -V
to ground. Connect SHDN to IN or
in Skip mode with external resistors
(Output Voltage Selection)
= - V
depends on the accuracy of the
REF
REF
OUT
IN
/ 50µA
R2
R1
/ 50µA
Resistor Selection
or some other positive
FLY
Shutdown Mode
charges faster due
IN
is an unregulat-
Skip Mode
Charge-Pump DC-DC Inverter
IN
in LIN
Regulated, 125mA-Output,
A C
specified load current. However, for minimum ripple in
Skip mode, this value may need to be increased.
Maxim recommends 2.2µF.
Surface-mount ceramic capacitors are preferred for
C
lent series resistance (ESR). To ensure proper opera-
tion over the entire temperature range, choose ceramic
capacitors with X7R (or equivalent) low-temperature-
coefficient (tempco) dielectrics. See Table 1 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 ten times greater than that
of the flying capacitor.
When in Skip mode, output ripple depends mostly on
two parameters: charge transfer between the capaci-
tance values of C
The ESR ripple contribution occurs as C
The charging current creates a negative voltage pulse
across the capacitor’s ESR that recedes as C
charges. At equilibrium, when the voltage on C
approaches that on C
Secondly, the ripple contribution due to charge transfer
between capacitors creates a pulse as charge flows to
C
to-peak ripple because their peaks do not occur at the
same time. It is best to use only the dominant term. The
expression for the ripple component predominantly due
to C
Figure 4. Separate V
FLY
OUT
INPUT
FLY
OUT
5.0V
LIN
ON
, due to their small size, low cost, and low equiva-
. Adding the two terms does not determine peak-
2.2 F
C
value of 1µF or more is sufficient to supply the
FLY
ESR is:
10 F
C
IN
SKIP
OFF
4
2
3
1
FLY
REF
SHDN
CAP+
CAP-
LIN/SKIP
and C
for Voltage Divider
OUT
MAX1673
IN
8
, no charging current flows.
OUT
Capacitor Selection
GND
OUT
FB
, and the ESR of C
7
6
5
60.4k
100k
R1
R2
22 F
C
V
OUT
5V
REF
OUT
V
OUT
= -V
charges.
REF
OUTPUT
-3V
x R2
OUT
OUT
R1
FLY
7
.