Rainbow Electronics MAX653 - Manual (Page 8)

MAX639/MAX640/MAX653
Figure 2 shows what happens to the ideal circuit of
Figure 1 if the switch turns on with a 66% duty cycle and
V+ = 3/2 V
OUT . The inductor current rises more slowly
than it falls because the magnitude of the voltage
applied during t
ON is less than that applied during t OFF .
Varying the duty cycle and switching frequency keeps
the peak current constant as input voltage varies. The
MAX639/MAX640/MAX653 control the switch (t
ON and
t OFF ) according to the following equations:
Equation (1) t ON = 50µsV / (V+ - V OUT )
Equation (2) t OFF
≥
50µsV / V OUT
Equation (3) I PEAK = 50µsV / L
These three equations ensure constant peak currents for
a given inductor value, across all input voltages (ignoring
the voltage drop across the diode (D1) and the resistive
losses in the switch and inductor). The variable duty
cycle also ensures that the current through the inductor
discharges to zero at the end of each pulse.
Figure 3 shows the MAX639/MAX640/MAX653 block dia-
gram and a typical connection in which 9V is converted
to 5V (MAX639), 3.3V (MAX640), or 3.0V (MAX653). The
sequence of events in this application is as follows:
When the output dips:
(1) The error comparator switches high.
(2) The internal oscillator starts (15µs start-up time)
and connects to the gate of the LX output driver.
(3) LX turns on and off according to t ON and t OFF ,
charging and discharging the inductor, and sup-
plying current to the output (as described above).
When the output voltage recovers:
(1) The comparator switches low.
(2) LX turns off.
(3) The oscillator shuts down to save power.
Fixed or Adjustable Output
For operation at the preset output voltage, connect VFB
to GND; no external resistors are required. For other
output voltages, use an external voltage divider. Set the
output voltage using R3 and R4 as determined by the
following formula:
R3 = R4 [(V OUT / VFB Threshold) - 1]
where R4 is any resistance in the 10k
Ω
to 1M
Ω
range (typ-
ically 100k
Ω
), and the VFB threshold is typically 1.28V.
5V/3.3V/3V/Adjustable, High-Efficiency,
Low I
Q
, Step-Down DC-DC Converters
8 _______________________________________________________________________________________
MAX639
MAX640
MAX653
VARIABLE
FREQUENCY
AND
DUTY-CYCLE
OSCILLATOR
ERROR
COMPARATOR
LOW-BATTERY
COMPARATOR
+1.28V
BANDGAP
REFERENCE
MODE-SELECT
COMPARATOR
C
OUT
100 µ F
1N5817
L = 100 µ H
5V, 3.3V OR 3.0V
AT 100mA
5
1
LX
VOUT
50mV
GND
4
VFB
7
R2
2
3
LBO
LBI
R1
C IN
33 µ F
INPUT, +5.5V TO +11.5V (MAX639),
+3.8V TO +11.5V (MAX640), +3.5V TO +11.5V (MAX653)
8 6
SHDN V+
Figure 3. Block Diagram