MAX15004_11 Datasheet, PDF (12/27 Pages) Maxim Integrated Products – 4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers

4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
operation of the power supply; however, to reduce the
power dissipation of the internal LDO, it can be dis-
abled by applying an external voltage higher than 7.4V
at V_CC (LDO output). The LDO then stops drawing cur-
rent from IN, thereby reducing the power dissipation in
the IC. The V_CC voltage is clamped to 10.4V with 30mA
current sink in case there is a higher voltage at the bias
winding. This feature is useful in applications with con-
tinuous higher input voltage.
The second 5V LDO regulator from V_CC to REG5 pro-
vides power to the internal control circuits. This LDO can
also be used to source 15mA of external load current.
Bypass V_CC and REG5 with a parallel combination of
1μF and 0.1μF low-ESR ceramic capacitors. Additional
capacitors (up to 22μF) at V_CC can be used although
they are not necessary for proper operation of the
MAX15004/MAX15005.
Startup Operation/UVLO/ON/OFF
The MAX15004A/B/MAX15005A/B feature two under-
voltage lockouts (UVLO). The internal UVLO monitors
the V_CC-regulator and turns on the converter once V_CC
rises above 3.5V. The internal UVLO circuit has about
0.5V hysteresis to avoid chattering during turn-on. Once
the power is on and the bootstrapped voltage feeds
V_CC, IN voltage can drop below 4V. This feature pro-
vides operation at a cold-crank voltage as low as 2.5V.
An external undervoltage lockout can be achieved by
controlling the voltage at the ON/OFF input. The
ON/OFF input threshold is set at 1.23V (rising) with
75mV hysteresis.
Before any operation can commence, the ON/OFF volt-
age must exceed the 1.23V threshold.
Calculate R1 in Figure 1 by using the following formula:
R1 =
⎛
⎝⎜
V_ON
V_UVLO
⎞
− 1_⎠⎟
× R2
where V_UVLO is the ON/OFF’s 1.23V rising threshold,
and V_ON is the desired input startup voltage. Choose
an R2 value in the 100kΩ range. The UVLO circuits
keep the PWM comparator, ILIM comparator, oscillator,
and output driver shut down to reduce current con-
sumption (see the Functional Diagram). The ON/OFF
input can be used to disable the MAX15004/MAX15005
and reduce the standby current to less than 20μA.
Soft-Start
The MAX15004/MAX15005 are provided with an exter-
nally adjustable soft-start function, saving a number of
external components. The SS is a 1.228V reference
bypass connection for the MAX15004A/B/MAX15005A/B
MAX15004A/B
MAX15005A/B
1.23V
ON/OFF
VIN
R1
R2
Figure 1. Setting the MAX15004A/B/MAX15005A/B
Undervoltage Lockout Threshold
and also controls the soft-start period. At startup, after
V_IN is applied and the UVLO thresholds are reached,
the device enters soft-start. During soft-start, 15μA is
sourced into the capacitor (C_SS) connected from SS to
GND causing the reference voltage to ramp up slowly.
The HICCUP mode of operation is disabled during soft-
start. When V_SS reaches 1.228V, the output as well as
the HICCUP mode become fully active. Set the soft-start
time (t_SS) using following equation:
t _SS
=
1.23(V) × C_SS
15 × 10^−6 (A)
where t_SS is in seconds and C_SS is in farads.
The soft-start programmability is important to control the
input inrush current issue and also to avoid the
MAX15004/MAX15005 power supply from going into the
unintentional hiccup during the startup. The required
soft-start time depends on the topology used, current-
limit setting, output capacitance, and the load condition.
Oscillator Frequency/
External Synchronization
Use an external resistor and capacitor at RTCT to pro-
gram the MAX15004A/B/MAX15005A/B internal oscillator
frequency from 15kHz to 1MHz. The MAX15004A/B out-
put switching frequency is one-half the programmed
oscillator frequency with a 50% maximum duty-cycle
limit. The MAX15005A/B output switching frequency is
the same as the oscillator frequency. The RC network
connected to RTCT controls both the oscillator frequency
and the maximum duty cycle. The CT capacitor charges
and discharges from (0.1 x V_REG5) to (0.55 x V_REG5). It
charges through RT and discharges through an internal
trimmed controlled current sink. The maximum duty
cycle is inversely proportional to the discharge time
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