Rainbow Electronics MAX15041 - Manual (Page 10)

MAX15041
Low-Cost, 3A, 4.5V to 28V Input, 350kHz, PWM
Step-Down DC-DC Regulator with Internal Switches
10 ______________________________________________________________________________________
Programmable Soft-Start (SS)
The MAX15041 utilizes a soft-start feature to slowly ramp
up the regulated output voltage to reduce input inrush
current during startup. Connect a capacitor from SS to
SGND to set the startup time (see the
Setting the Soft-
Start Time section for capacitor selection details ) .
Internal LDO (V DD )
The MAX15041 has an internal 5.1V (typ) LDO. V DD is
externally compensated with a minimum 1µF, low-ESR
ceramic capacitor. The V
DD
voltage is used to supply
the low-side MOSFET driver, and to supply the internal
control logic. When the input supply (IN) is below 4.5V,
V
DD is 50mV (typ) lower than IN. The V DD output cur-
rent limit is 80mA (typ) and an UVLO circuit inhibits
switching when V
DD falls below 3.85V (typ).
Error Amplifier
A high-gain error amplifier provides accuracy for the volt-
age feedback loop regulation. Connect the necessary
compensation network between COMP and SGND (see
the
Compensation Design Guidelines section). The error-
amplifier transconductance is 1.6mS (typ). COMP clamp
low is set to 0.68V (typ), just below the PWM ramp com-
pensation valley, helping COMP to rapidly return to cor-
rect set point during load and line transients.
PWM Comparator
The PWM comparator compares COMP voltage to the
current-derived ramp waveform (LX current to COMP volt-
age transconductance value is 9A/V, typ.). To avoid insta-
bility due to subharmonic oscillations when the duty cycle
is around 50% or higher, a compensation ramp is added
to the current-derived ramp waveform. The compensation
ramp slope (0.45V x 350kHz) is equivalent to half of the
inductor current down slope in the worst case (load 3A,
current ripple 30% and maximum duty cycle operation of
90%). Compensation ramp valley is set at 0.83V (typ).
Overcurrent Protection
and Hiccup Mode
When the converter output is shorted or the device is
overloaded, the high-side MOSFET current-limit event
(6A, typ) turns off the high-side MOSFET and turns on
the low-side MOSFET. In addition, it discharges the SS
capacitor, C SS for a fixed period of time ( ∆T
0 = 70ns,
typ). If the overcurrent condition persists, SS is pulled
below 0.606V and a hiccup event is triggered.
During a hiccup event, high-side and low-side
MOSFETs are kept off, and COMP is pulled low for a
period equal to 16 times the nominal soft-start time
(blanking time). This is obtained by charging SS from 0
to 0.606V with a 5µA (typ) current, and then slowly dis-
charging it back to 0V with a 333nA (typ) current. After
the blanking time has elapsed, the device attempts to
restart. If the overcurrent fault has cleared, the device
resumes normal operation, otherwise a new hiccup
event is triggered (see the Output Short-Circuit
Waveforms in the
Typical Operating Characteristics ).
Thermal-Shutdown Protection
The MAX15041 contains an internal thermal sensor that
limits the total power dissipation in the device and pro-
tects it in the event of an extended thermal fault condi-
tion. When the die temperature exceeds +155°C (typ),
the thermal sensor shuts down the device, turning off
the DC-DC converter and the LDO regulator to allow
the die to cool. After the die temperature falls by 20°C
(typ), the device restarts, using the soft-start sequence.
Applications Information
Setting the Output Voltage
Connect a resistive divider (R 1 and R 2 , see Figures 1
and 2) from OUT to FB to SGND to set the DC-DC con-
verter output voltage. Choose R
1 and R 2 so that the DC
errors due to the FB input bias current do not affect the
output-voltage precision. With lower value resistors, the
DC error is reduced, but the amount of power consumed
in the resistive divider increases. A typical tradeoff value
for R
2 is 10k
Ω, but values between 5kΩ and 50kΩ are
acceptable. Once R 2 is chosen, calculate R 1 using:
where the feedback threshold voltage V FB = 0.606V
(typ).
R R
V
V
OUT
FB
1 2
1
= Ч −
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