Rainbow Electronics MAX747 - Manual (Page 8)

With decreasing loads, as the inductor current becomes
discontinuous, traditional PWM converters continue to
switch at a fixed frequency, decreasing light-load efficiency.
However, the MAX747’s idle-mode comparator increases
the peak inductor current, allowing more energy to be
transferred per cycle. Since fewer cycles are required, the
switching frequency is reduced. This keeps the external P-
FET off for longer periods, minimizing switching losses and
increasing efficiency.
The light-load output noise spectrum widens due to variable
switching frequency in idle-mode, but output ripple remains
low. Using the
Typical Operating Circuit, with a 9V input and
a 125mA load current, output ripple is less than 40mV.
Soft-Start and Current Limiting
The MAX747 draws its highest current at power-up. If The
power source to the MAX747 cannot provide this initial
elevated current, the circuit may not function correctly. For
example, after prolonged use, a battery’s increased series
resistance may prevent it from providing adequate initial
surge currents when the MAX747 is brought out of
shutdown. Using Soft-Start (SS) minimizes the possibility of
overloading the incoming supply at power-up by gradually
increasing the peak current limit. Connect an external
capacitor from SS to ground to reduce the initial peak
currents drawn from the supply.
The steady-state SS pin voltage is typically 3.8V. On
power-up, SS sources 1µA until the SS voltage reaches
3.8V. The current-limit comparator inhibits EXT switching
until the SS voltage reaches 1.8V. The maximum current
limit is set by:
Figure 3 shows how the peak current limit increases as
the voltage on SS rises for two R
SENSE
values.
Shutdown Mode
When SHDN is high, the MAX747 enters shutdown
mode. In this mode, the internal biasing circuitry
(including EXT) is turned off, V
OUT
drops to 0V, and the
supply current drops to 0.6µA (20µA max). This
excludes external component leakage, which may add
several microamps to the shutdown supply current for
the entire circuit. SHDN is a TTL/CMOS logic-level
input. Connect SHDN to GND for normal operation.
Low-Battery Detector
The MAX747 provides a low-battery comparator that
compares the voltage on LBI to the reference voltage.
LBO, an open-drain output, goes low when the LBI
voltage is below V
REF
. Use a resistor-divider network
as shown in Figure 4 to set the trip voltage (V
TRIP
) to
the desired level. In this circuit, LBO goes low when
V+
≤
V
TRIP
. LBO is high impedance in shutdown mode.
__________________Design Procedure
Setting the Output Voltage
The MAX747’s output voltage can be set to 5V by
grounding FB, or adjusted from 2V to 14V using
external resistors R4 and R5, configured as shown in
Figure 5. Select feedback resistor R4 from the 10k
Ω
to
1M
Ω
range. R5 is given by:
Selecting R SENSE
First, approximate the peak current assuming I
PK
is
(1.1)(I
LOAD
), where I
LOAD
is the maximum load current.
Once all component values have been determined, the
actual peak current is given by:
Next, determine the value of R
SENSE
such that:
For example, to obtain 5V at 3A, I
PK
= 3.3A and R
SENSE
=
125mV/3.3A = 38m Ω .
The sense resistor should have a power rating greater
than (I
PK
2 )(R
SENSE
) (with an adequate safety margin).
With a 3A load current, I
PK
= 3.3A and R
SENSE
= 38m
Ω
.
The power dissipated by the resistor (assuming an 80%
R
V
I
125mV
I
SENSE
LIMIT (MIN)
PK PK
= =
I I
V
(2L) (f )
1
V
V
PK LOAD
OUT
OSC
OUT
IN
= +





 −

 

R5 (R4)
V
2V
1
OUT = −






I
V
R
150mV (typ)
R
PK
LIMIT
SENSE SENSE
= =
MAX747
High-Efficiency PWM, Step-Down
P-Channel DC-DC Controller
8 _______________________________________________________________________________________
0
MAX747-FIG3
SOFT-START VOLTAGE (V)
PEAK CURRENT LIMIT (A)
4
2
1
1 3
3
2
0
R SENSE = 50m Ω
R SENSE = 100m Ω
V+ –V CS = 150mV
Figure 3. Peak Current Limit vs. Soft-Start Voltage