Description
AXPM11585 is a high efficiency and fast synchronous stepdown DC-DC converter capable of delivering
5A output current. It operates over a wide input voltage range from 4.5V to 30V and integrates main switch
and synchronous switch with very low Rds(on) to minimize the conduction loss. AXPM11585 adopts the
instant PWM architecture to achieve fast transient responses for high step-down applications and high
efficiency at light loads. In addition, it operates at pseudo-constant frequency of 500kHz under continuous
conduction mode to minimize the size of inductor and capacitor.
Features
▪ 95% efficiency at 1A 5V output
▪ Low Rds(on) for internal switches (top/bottom): 70/40mΩ
▪ 4.5-30V input voltage range
▪ Instant PWM architecture to achieve fast transient responses
▪ External soft start limits the inrush current
▪ Pseudo-constant frequency: 500kHz at heavy loads
▪ 5A continuous, 6A peak load current capability
▪ 1.5% 0.6V reference
▪ Output over current limit
▪ Output short circuit protection with current foldback
▪ Thermal shutdown and auto recovery
▪ RoHS Compliant and Halogen Free
Applications
▪ Automotive systems
▪ LCD TV
▪ Set Top Box
▪ Notebook
▪ Storage
▪ High power AP router
▪ Networking
1 Block Diagram and Application Circuit
2 Pin Description
2.1 Pin Names
2.2 Pin Functions
3. Electrical Specifications
3.1 Absolute Maximum Ratings
3.2 Thermal Data
3.3 ESD and Latch Up
3.4 Electrical Characteristics
VIN = 12V; VEN = 2.5V; VOUT = 5V; Tamb = +25°C; unless otherwise specified.
4 Functional Description
4.1 Overview
AXPM11585 is a synchronous buck regulator IC that integrates the PWM control, top and
bottom switches on the same die to minimize the switching transition loss and conduction loss.
With ultra-low Rds(on) power switches and efficient PWM control, this regulator IC can achieve
the highest efficiency and the highest switch frequency simultaneously to minimize the external
inductor and capacitor size, and thus achieving the minimum solution footprint. AXPM11585
provides protection functions such as cycle by cycle current limiting and thermal shutdown
protection. It will sense the output voltage conditions for the fault protection.
4.2 PWM Control
Due to the high integration in the AXPM11585, the application circuit based on this regulator
IC is rather simple. Only input capacitor CVIN, output capacitor COUT, output inductor L1 and
feedback resistors (R1 and R2) need to be selected for the targeted applications. AXPM11585
integrates the compensation components to achieve good stability and fast transient
responses.
4.3 Internal VCC Regulator
There is a 5V reference, VCC, internally generated, for powering internal analog and drive
circuits. This pin should be bypassed to ground with a 1μF ceramic capacitor. This pin may be
used with an external DC load of 20mA or less.
4.4 Enable Control
AXPM11585 has a dedicated enable control pin (EN). With high enough input voltage, the
device can be enabled and disabled by EN which has positive logic. Its falling threshold is 1.2V.
When EN is pulled down below 1.2V, the device is put into the lowest shut down current mode
of <4μA. By means of external RC connected to EN pin, it Is possible to program the delay turn
on time upon driving EN high.
4.5 Under Voltage Lockout (UVLO)
Under voltage lockout (UVLO) is implemented to protect the device from operating at
insufficient supply voltage. The UVLO rising threshold is about 4V with 0.2V hysteresis.
4.6 Soft start
The soft start is implemented to prevent the converter output voltage from overshooting during
start up. Soft start is programmed by connecting a capacitor from soft start programming pin
to ground. Tss=Css*0.6V/10μA.
4.7 Thermal Shutdown
Thermal shutdown is implemented to prevent the device from operating at exceedingly high
temperatures. When the silicon die temperature is higher than its upper threshold, it shuts
down the whole device. When the temperature is lower than its lower threshold, the device is
enabled again.
4.8 Bootstrap
The power MOSFET driver is powered by an external bootstrap capacitor. The bootstrap
capacitor is charged and regulated to about 5V by the dedicated internal bootstrap regulator.
The BS capacitor provides the gate driver voltage for internal high side MOSEFET. A 100nF
low ESR ceramic capacitor connected between BS pin and LX pin is recommended.
4.9 Internal VCC LDO
There is a 5V reference, VCC, internally generated. This pin should be bypassed to ground
with a 1μF ceramic capacitor. This pin may be used with an external DC load of 20mA or less.
5 Components Selection
5.1 Setting the Output Voltage
The output voltage is set using a resistive voltage divider from the output voltage to FB pin. It
is given as follows:
5.2 Inductor
There are several considerations in choosing the inductor.
1) Choose the inductance to provide the desired ripple current. It is suggested to choose the
ripple current to be about 40% of the maximum output current. The inductance is calculated
as:
where Fsw is the switching frequency and Ioutmax is the maximum load current. The
AXPM11585 regulator IC is quite tolerant of different ripple current amplitude.
Consequently, the final choice of inductance can be slightly off the calculation value without
significantly impacting the performance.
2) The saturation current rating of the inductor must be selected to be greater than the peak
inductor current under full load conditions.
3) The DCR of the inductor and the core loss at the switching frequency must be low enough
to achieve the desired efficiency requirement. It is desirable to choose an inductor with
DCR<10mΩ to achieve a good overall efficiency.
5.3 Input Capacitor
To minimize the potential noise problem, place a typical X5R or better grade ceramic capacitor
close to the VIN and GND pins. Care should be taken to minimize the loop area formed by
CVIN, and VIN/GND pin. 10μF low ESR ceramic capacitor is recommended.
5.4 Output Capacitor
The output capacitor is selected to handle the output ripple noise requirements. Both steady
state ripple and transient requirements must be taken into consideration when selecting this
capacitor. For the best performance, it is recommended to use X5R or better grade ceramic
capacitor greater than 47μF capacitance.