Re g u la t e d , 1 2 5 m A-Ou t p u t ,
Ch a rg e -P u m p DC-DC In ve rt e r
MAX1673
Lin e a r Mo d e (Co n s t a n t -Fre q u e n c y Mo d e )
V
5V
REF
In LIN mode (LIN/SKIP = IN), the charge pump runs con-
tinuously at 350kHz. The MAX1673 controls the charge
INPUT
5.0V
C
10µF
IN
R1
100k
on C
by varying the gate drive on S1 (Figure 2).
FLY
8
When the output voltage falls, C
charges faster due
FLY
ON
6
4
2
R2
R1
IN
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).
V
OUT
= -V
REF
x
OFF
SHDN
CAP+
FB
R2
60.4k
MAX1673
C
FLY
2.2µF
5
OUTPUT
-3V
OUT
3
1
CAP-
C
OUT
LIN
22µF
LIN/SKIP
GND
7
SKIP
S k ip Mo d e
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.
Figure 4. Separate V
for Voltage Divider
REF
Ca p a c it o r S e le c t io n
value of 1µF or more is sufficient to supply the
specified load current. However, for minimum ripple in
Skip mod e , this va lue ma y ne e d to b e inc re a s e d .
Maxim recommends 2.2µF.
S h u t d o w n Mo d e
When SHDN (a CMOS-compatible input) is driven low,
the MAX1673 e nte rs low-p owe r s hutd own mod e .
Charge-pump switching action halts and an internal 1Ω
A C
FLY
switch pulls V
to ground. Connect SHDN to IN or
OUT
drive high for normal operation.
Surface-mount ceramic capacitors are preferred for
C
, due to their small size, low cost, and low equiva-
FLY
*See Output Ripple vs. Load Current in Typical Operating Characteristics.
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.
Ap p lic a t io n s In fo rm a t io n
Re s is t o r S e le c t io n
(Ou t p u t Vo lt a g e S e le c t io n )
The accuracy of V
depends on the accuracy of the
OUT
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.
voltage biasing the voltage-divider network (R1, R2).
Use a separate reference voltage if V is an unregulat-
IN
ed voltage or if greater accuracy is desired (Figure 4).
Adjust the output voltage from -1.5V to -V in LIN
IN
mode or 0V to -V in Skip mode with external resistors
IN
When in Skip mode, output ripple depends mostly on
two parameters: charge transfer between the capaci-
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:
tance values of C
and C
, and the ESR of C
OUT
.
FLY
OUT
The ESR ripple contribution occurs as C
charges.
OUT
The charging current creates a negative voltage pulse
a c ros s the c a p a c itor’s ESR tha t re c e d e s a s C
OUT
R2
V
OUT
= -V
REF
charges. At equilibrium, when the voltage on C
FLY
R1
approaches that on C
, no charging current flows.
OUT
where V
reference source.
can be either V or some other positive
Secondly, the ripple contribution due to charge transfer
between capacitors creates a pulse as charge flows to
REF
IN
C
. Adding the two terms does not determine peak-
OUT
Typically, choose a voltage-divider current of 50µA to
minimize the effect of FB input current:
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
R1 = V
/ 50µA
REF
to C
ESR is:
OUT
R2 = -V
/ 50µA
OUT
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7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
