Rainbow Electronics MAX8575 - Manual (Page 7)

MAX8570–MAX8575
High-Efficiency LCD Boost
with True Shutdown
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Detailed Description
The MAX8570 family of compact, step-up DC-DC con-
verters operate from a 2.7V to 5.5V supply. Consuming
only 25µA of supply current, these ICs include an inter-
nal MOSFET switch with a low on-resistance. A true-
shutdown feature disconnects the battery from the load
and reduces the supply current to 0.05µA (typ). These
DC-DC converters are available with either a fixed 15V
output or are adjustable up to 28V. Three current-limit
options are available: 110mA, 250mA, and 500mA. See
the Selector Guide on page 1.
Control Scheme
The MAX8570 family features a minimum off-time cur-
rent-limited control scheme operating in discontinuous
mode. An internal p-channel MOSFET switch connects
V
CC
to SW to provide power to the inductor when the
converter is operating. When the converter is shut
down, this switch disconnects the input supply from the
inductor (see Figure 1).
To boost the output voltage, an n-channel MOSFET
switch turns on and allows current to ramp up in the
inductor. Once this current reaches the current limit,
the switch turns off and the inductor current flows
through D1 to supply the output. The switching fre-
quency varies depending on the load and input voltage
and can be up to 800kHz.
Setting the Output Voltage
The output voltage of the MAX8570, MAX8571, and
MAX8574 is adjustable from V
CC to 28V by using a
resistor voltage-divider (see Figure 2). Select R2 from
10k
Ω to 600kΩ and calculate R1 with the following
equation:
where V FB = 1.226V and V OUT can range from V CC to
28V. For best accuracy, ensure that the bias current
through the feedback resistors is at least 2µA.
The MAX8572, MAX8573, and MAX8575 have a fixed
15V output. When using these parts, connect OUT
directly to the output (see Figure 3).
Shutdown (
SHDN
)
Drive SHDN low to enter shutdown. During shutdown
the supply current drops to 0.05µA (typ), the output is
disconnected from the input, and LX enters a high-
impedance state. The capacitance and load at the out-
put determine the rate at which V
OUT decays. SHDN
can be pulled as high as 6V regardless of the input and
output voltages.
With a typical step-up converter circuit, the output
remains connected to the input through the inductor
and output rectifier, holding the output voltage to one
diode drop below V
CC when the converter is shut down
and allowing the output to draw power from the input.
The MAX8570 family features true-shutdown mode, dis-
connecting the output from the input with an internal p-
channel MOSFET switch when shut down. This
eliminates power draw from the input during shutdown.
Soft-Start
The MAX8570 family uses two soft-start mechanisms.
When the true-shutdown feature is used (SW is con-
nected as in Figure 2 and Figure 3), the gate of the
internal high-side p-channel switch turns on slowly to
prevent inrush current. This takes approximately 200µs.
When SW is fully turned on, the internal n-channel
switch begins boosting the input to set the output volt-
age. When V
FB is less than 0.5V (with or without the use
of true shutdown), the minimum off-time of the internal
n-channel switch increases from 1µs to 5µs to control
inrush current.
Separate Power for Inductor
Separate power supplies can be used for the IC and
the inductor. This allows power to be used from a bat-
tery or supply with a voltage as low as 0.8V, or higher
than the V
CC operating range of the converter. When
using a separate inductor supply, SW is left unconnect-
ed and the supply is connected directly to the inductor
(see Figure 4). Note that in this configuration the output
is no longer disconnected from the input during shut-
down. In shutdown the output voltage goes to a diode
drop below the inductor supply voltage.
Protection Features
The MAX8570 family has protection features designed
to make it extremely robust to application errors (see
Table 1). If the output capacitor in the application is
missing, the MAX8570 family protects the internal
switch from being damaged. If the top feedback resis-
tor or the external diode is disconnected, the converter
stops switching and the output is resistively loaded to
ground. Similarly, if the external diode polarity is
reversed, the converter discontinues switching. If the
bottom feedback resistor is missing, the output stays at
a diode drop less than the inductor supply voltage or
1.226V (whichever is greater). In fact, in response to
most fault conditions, the MAX8570 family protects not
only itself, but also the downstream circuitry.
R R
V
V
OUT
FB
1 2 1
= −
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