MAX1536 Datasheet, PDF (15/19 Pages) Maxim Integrated Products – 3.6A, 1.4MHz, Low-Voltage, Internal-Switch Step-Down Regulator with Dynamic Output Voltage Control

3.6A, 1.4MHz, Low-Voltage, Internal-Switch Step-
Down Regulator with Dynamic Output Voltage Control
R_TOFF
=
(t_OFF
−
0.07μs)
110kΩ
1.00μs
V_TOFF is typically 1.1V and the recommended values
for R_TOFF range from 30.1kΩ to 499kΩ for off-times of
0.3µs to 4.5µs.
Frequency Variation with Output Current
The operating frequency of the MAX1536 in PWM mode
is determined primarily by t_OFF (set by R_TOFF), V_IN, and
V_OUT as shown in the following formula:
f_PWM
=
V_IN - V_OUT - V_PMOS
t_OFF (V_IN - V_PMOS + V_NMOS)
where:
V_PMOS = the voltage drop across the internal PMOS
power switch, I_OUT × R_PMOS.
V_NMOS = the voltage drop across the internal NMOS
synchronous-rectifier switch, I_OUT × R_NMOS.
As the output current increases, V_NMOS and V_PMOS
increase and the voltage across the inductor decreas-
es. This causes the frequency to drop. Approximate the
change in frequency with the following formula:
Δf_PWM
=
-
I_OUT x R_PMOS
V_IN x t_OFF
where R_PMOS is the resistance of the internal MOSFETs
(54mΩ, typ).
Inductor Selection
The key inductor parameters must be specified: induc-
tor value (L) and peak current (I_PEAK). The following
equation includes a constant, denoted as LIR, which is
the ratio of peak-to-peak inductor AC ripple current to
maximum DC load current. A higher value of LIR allows
smaller inductance but results in higher losses and rip-
ple. A good compromise between size and losses is
found at approximately a 25% ripple-current to load-
current ratio (LIR = 0.25), which corresponds to a peak-
inductor current 1.125 times the DC load current:
L = ^{VOUT x tOFF}
I_OUT x LIR
where:
I_OUT = maximum DC load current.
LIR = ratio of peak-to-peak AC inductor current to DC
load current, typically 0.25.
The peak-inductor current at full load is 1.125 × I_OUT if
the above equation is used; otherwise, the peak current
is calculated by:
I_PEAK
= I_OUT
+
V_OUT x t_OFF
2xL
Choose an inductor with a saturation current at least as
high as the peak-inductor current. The inductor select-
ed should exhibit low losses at the chosen operating
frequency.
Capacitor Selection
The input-filter capacitor reduces peak currents and
noise at the voltage source. Use a low-ESR and low-
ESL capacitor located no further than 5mm from IN.
Select the input capacitor according to the RMS input
ripple-current requirements and voltage rating:
I_RIPPLE = I_LOAD
V_OUT (V_IN - V_OUT)
V_IN
where I_RIPPLE = input RMS current ripple.
The output-filter capacitor affects the output-voltage
ripple, output load-transient response, and feedback-
loop stability. For stable operation, the MAX1536
requires a minimum output ripple voltage of V_RIPPLE ≥
1% × V_OUT.
The minimum ESR of the output capacitor is calculated by:
ESR >1% x ^L
t_OFF
Stable operation requires the correct output-filter capaci-
tor. When choosing the output capacitor, ensure that:
C_OUT
≥
t_OFF
V_OUT
x
79μF x 1V
1μs
Integrator Amplifier
An internal transconductance amplifier fine tunes the
output DC accuracy. A capacitor, C_COMP, from COMP
to V_CC compensates the transconductance amplifier.
For stability, choose C_COMP = 470pF. A larger capaci-
tor value maintains a constant average output voltage,
but slows the loop response to changes in output volt-
age. A smaller capacitor value speeds up the loop
response to changes in output voltage but decreases
stability.
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