MAX1845 Maxim, MAX1845 Datasheet - Page 21

• Manufacturer Part Number: MAX1845
• Description: Dual / High-Efficiency / Step-Down Controller with Accurate Current Limit
• Manufacturer: Maxim

The output voltage adjust range for continuous-conduc-
tion operation is restricted by the nonadjustable 500ns
(max) minimum off-time one-shot. For best dropout per-
formance, use the slower on-time settings. When work-
ing with low input voltages, the duty-cycle limit must be
calculated using the worst-case values for on- and off-
times. Manufacturing tolerances and internal propaga-
tion delays introduce an error to the TON K-factor. This
error is greater at higher frequencies (Table 4). Also,
keep in mind that transient response performance of
buck regulators operating close to dropout is poor, and
bulk output capacitance must often be added (see the
V
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (∆I
as much as it ramps up during the on-time (∆I
ratio h = ∆I
the inductor current higher in response to increased
load and must always be greater than 1. As h ap-
proaches 1, the absolute minimum dropout point, the
inductor current will be less able to increase during each
switching cycle, and V
additional output capacitance is used.
A reasonable minimum value for h is 1.5, but this may
be adjusted up or down to allow trade-offs between
V
voltage. For a given value of h, calculate the minimum
operating voltage as follows:
where V
drops in the discharge and charge paths (see the On-
Time One-Shot (TON) section), t
Electrical Characteristics, and K is taken from Table 4.
The absolute minimum input voltage is calculated with h
= 1.
If the calculated V
minimum input voltage, then reduce the operating fre-
quency or add output capacitance to obtain an accept-
able V
calculate V
V
SAG
SAG
IN(MIN)
, output capacitance, and minimum operating
equation in the Design Procedure section).
SAG
DROP1
= [(V
. If operation near dropout is anticipated,
SAG
UP
+ V
/ ∆I
OUT
DROP2
Applications Information
to ensure adequate transient response.
and V
DOWN
______________________________________________________________________________________
IN(MIN)
+ V
- V
DROP2
Controller with Accurate Current Limit
DROP1
SAG
is an indicator of ability to slew
DROP1
is greater than the required
Dropout Performance
will greatly increase unless
) / {1 - (t
are the parasitic voltage
OFF(MIN)
Dual, High-Efficiency, Step-Down
OFF(MIN)
is from the
✕
UP
h / K)}]
DOWN
). The
)
Dropout Design Example:
V
fsw = 600kHz
K = 1.63µs, worst-case K = 1.4175µs
t
V
h = 1.5
Calculating again with h = 1 gives an absolute limit of
dropout:
Therefore, V
very large output capacitance, and a practical input
voltage with reasonable output capacitance would be
3.8V.
The MAX1845’s dual-mode operation allows the selec-
tion of common voltages without requiring external
components (Figure 8). Connect FB1 to GND for a fixed
1.8V output or to V
directly to OUT1 for a fixed 1V output.
Connect FB2 to GND for a fixed 2.5V output or to OUT2
for a fixed 1V output.
The output voltage can be adjusted from 1V to 5.5V
with a resistor-divider network (Figure 9). The equation
for adjusting the output voltage is:
where V
Careful PC board layout is critical to achieve low
switching losses and clean, stable operation. This is
especially true for dual converters, where one channel
can affect the other. The switching power stages
require particular attention (Figure 10). Refer to the
MAX1845 evaluation kit data sheet for a specific layout
example.
Use a four-layer board. Use the top side for power
components and the bottom side for the IC and the
sensitive ground components. Use the two middle lay-
ers as ground planes, with interconnections between
the top and bottom layers as needed. If possible,
OFF(MIN)
V
OUT
DROP1
V
IN(MIN)
IN(MIN)
= 1.8V
Setting V
FB
= V
= (1.8V + 0.1V) / [1 - (0.5µs
= 500ns
_ is 1.0V and R2 is about 10kΩ.
= (1.8V + 0.1V) / [1 - (0.5µs
PC Board Layout Guidelines
+ 0.1V - 0.1V = 3.8V
DROP2
+ 0.1V - 0.1V = 2.8V
IN
V
OUT_
must be greater than 2.8V, even with
CC
OUT
= 100mV
=
for a 1.5V output, or connect FB1
V
_ with a Resistor-Divider
FB_
Fixed Output Voltages



1
+
R2
R1
✕



✕
1.5) / 1.4175µs]
1) / 1.4175µs]
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