Rainbow Electronics MAX1845 - Manual (Page 21)

MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
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Applications Information
Dropout Performance
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
SAG
equation in the Design Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (
∆I
DOWN
)
as much as it ramps up during the on-time ( ∆I
UP ). The
ratio h =
∆I
UP /
∆I
DOWN is an indicator of ability to slew
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
SAG will greatly increase unless
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
SAG , output capacitance, and minimum operating
voltage. For a given value of h, calculate the minimum
operating voltage as follows:
V
IN(MIN)
= [(V
OUT
+ V
DROP1
) / {1 - (t
OFF(MIN)
✕ h / K)}]
+ V DROP2 - V DROP1
where V
DROP1
and V
DROP2
are the parasitic voltage
drops in the discharge and charge paths (see the On-
Time One-Shot (TON) section), t
OFF(MIN) is from the
Electrical Characteristics, and K is taken from Table 4.
The absolute minimum input voltage is calculated with h
= 1.
If the calculated V IN(MIN) is greater than the required
minimum input voltage, then reduce the operating fre-
quency or add output capacitance to obtain an accept-
able V
SAG . If operation near dropout is anticipated,
calculate V SAG to ensure adequate transient response.
Dropout Design Example:
V OUT = 1.8V
fsw = 600kHz
K = 1.63µs, worst-case K = 1.4175µs
t OFF(MIN) = 500ns
V DROP1 = V DROP2 = 100mV
h = 1.5
V IN(MIN) = (1.8V + 0.1V) / [1 - (0.5µs ✕ 1.5) / 1.4175µs]
+ 0.1V - 0.1V = 3.8V
Calculating again with h = 1 gives an absolute limit of
dropout:
V IN(MIN) = (1.8V + 0.1V) / [1 - (0.5µs ✕ 1) / 1.4175µs]
+ 0.1V - 0.1V = 2.8V
Therefore, V IN must be greater than 2.8V, even with
very large output capacitance, and a practical input
voltage with reasonable output capacitance would be
3.8V.
Fixed Output Voltages
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
CC for a 1.5V output, or connect FB1
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.
Setting V OUT _ with a Resistor-Divider
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 FB _ is 1.0V and R2 is about 10k
Ω.
PC Board Layout Guidelines
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,
V V 1
R1
R2
OUT_ FB_
= +
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