MAX1549ETL Datasheet, PDF (22/35 Pages) Maxim Integrated Products – Dual, Interleaved, Fixed-Frequency Step-Down Controller with a Dynamically Adjustable Output

Dual, Interleaved, Fixed-Frequency Step-Down
Controller with a Dynamically Adjustable Output
Table 4. Open-Drain Output States
INPUTS
G1
G0
0
0
0
1
1
0
1
1
OD1
High-Z
0
High-Z
High-Z
Forced-PWM operation is required to ensure fast, accu-
rate negative voltage transitions when REFIN is low-
ered. Since forced-PWM operation disables the
zero-crossing comparator, the inductor current can
reverse under light loads, quickly discharging the out-
put capacitors. If fault blanking is enabled, the
MAX1549 disables the main controller’s (OUT1) output
fault protection (OVP and UVP), and forces PGOOD1 to
a high-impedance state for the period selected by
FBLANK (Table 5).
For a step voltage change at REFIN, the rate-of-change
of the output voltage is limited by the inductor current
ramp, the total output capacitance, the current limit, and
the load during the transition. The inductor current ramp
is limited by the voltage across the inductor and the
inductance. The total output capacitance determines
how much current is needed to change the output volt-
age. Additional load current slows down the output-volt-
age change during a positive REFIN voltage change,
and speeds up the output-voltage change during a neg-
ative REFIN voltage change. Increasing the current-limit
setting speeds up a positive output-voltage change.
OUTPUTS
OD2
High-Z
High-Z
0
High-Z
OD3
High-Z
High-Z
High-Z
0
R_EQ
R9
R9 // R12
R9 // R11
R9 // R10
Adding a capacitor across REFIN and GND filters noise
and controls the rate-of-change of the REFIN voltage
during dynamic transitions. With the additional capaci-
tance, the REFIN voltage slews between the two set
points with a time constant given by R_REFIN x C_REFIN,
where R_REFIN is the equivalent parallel resistance seen
by the slew capacitor during the transition:
τ_REFIN
=
⎛ _{R8 ×}
⎝⎜ ^{R8 +}
R_EQ
R_EQ
⎞
⎠⎟
C_REFIN
Dual-Mode Feedback (OUT2 Only)
The MAX1549’s dual-mode operation allows the selec-
tion of common voltages without requiring external
components (Figure 5). For the secondary controller
(OUT2), connect FB2 to GND for a fixed 2.5V output, to
V_CC for a fixed 1.8V output, or connect FB2 directly to
OUT2 for a fixed 0.5V output. The main controller
(OUT1) of the MAX1549 regulates to the voltage set at
REFIN (V_FB1 = V_REFIN) and does not support dual-
mode operation.
FB2
REF / 20
TO ERROR
AMPLIFIER
ADJUSTABLE
OUTPUT
MAX1549
OUT2
FIXED OUTPUT (1.8V)
FB = V_CC
FIXED OUTPUT (2.5V)
FB = GND
REF
Figure 5. Second Controller’s Dual-Mode Feedback
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