CMOS Micropower Step-Up
Switching Regulator
diode (D1) as well as that in the load. If the inductance
External Components
is too low, the current at L may exceed the maximum
X
Resistors
rating. The minimum allowed inductor value is
expressed by:
Since the LBR and V input bias currents are specified
FB
as 10nA (max), the current in the dividers R1/R2 and
R3/R4 (Figure 1) may be as low as 1µA without signifi-
cantly affecting accuracy. Normally R2 and R4 are
between 10kΩ and 1MΩ, which sets the current in the
voltage-dividers in the 1.3µA to 130µA range. R1 and
R3 can then be calculated as follows:
V
I
T
IN ON
L
MIN=
MAX
where I
≈ 525mA (peak L current) and t
is the
MAX
X
ON
on-time of the L output.
X
The most common MAX630 circuit is a boost-mode
converter (Figure 1). When the N-channel output device
is on, the current linearly rises since:
V
−1.31V
1.31
−1.31V
OUT
10Ω ≤ R2 ≤ 1MΩ R1= R2 x
V
LB
10Ω ≤ R4 ≤ 1MΩ R3 = R4 x
1.31
di
dt
V
L
=
where V
is the desired output voltage and V
the desired low-battery warning threshold.
is
LB
OUT
At the end of the on-time (14µs for 40kHz, 55% duty-
cycle oscillator) the current is:
If the I (shutdown) input is pulled up through a resistor
C
rather than connected directly to +V , the current
S
V T
L
5Vx14μs
470μH
ON
I
=150mA
through the pullup resistor should be a minimum of 4µA
pk=
=
with I at the input-high threshold of 1.3V:
C
+V −1.3V
The energy in the coil is:
S
R
≤
IC
4μA
2
LI
pk
E
=5.25μJ
=
Inductor Value
2
The available output current from a DC-DC voltage
boost converter is a function of the input voltage, exter-
nal inductor value, output voltage, and the operating
frequency.
At maximum load, this cycle is repeated 40,000 times
per second, and the power transferred through the coil
is 40,000 x 5.25 = 210mW. Since the coil only supplies
the voltage above the input voltage, at 15V, the DC-DC
converter can supply 210mW / (15V - 5V) = 21mA. The
coil provides 210mW and the battery directly supplies
another 105mW, for a total of 315mW of output power. If
the load draws less than 21mA, the MAX630 turns on its
output only often enough to keep the output voltage at
a constant 15V.
The inductor must 1) have the correct inductance, 2) be
able to handle the required peak currents, and 3) have
acceptable series resistance and core losses. If the
inductance is too high, the MAX630 will not be able to
deliver the desired output power, even with the L out-
X
put on for every oscillator cycle. The available output
power can be increased by either decreasing the
inductance or the frequency. Reducing the frequency
Reducing the inductor value increases the available
output current: lower L increases the peak current,
thereby increasing the available power. The external
inductor required by the MAX630 is readily obtained
from a variety of suppliers (Table 1). Standard coils are
suitable for most applications.
increases the on-period of the L output, thereby
X
increasing the peak inductor current. The available out-
put power is increased since it is proportional to the
square of the peak inductor current (I ).
PK
2
(V
T
) f
IN ON
L=
Types of Inductors
2P
OUT
Molded Inductors
These are cylindrically wound coils that look similar to
1W resistors. They have the advantages of low cost and
ease of handling, but have higher resistance, higher
losses, and lower power handling capability than other
types.
2
LI
f
pk
2
since: P
=
OUT
V
T
IN ON
L
and: I
pk=
where P
includes the power dissipated in the catch
OUT
6
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
