High-Efficiency, 2A, Current-Mode

Synchronous, Step-Down Switching Regulator

12 _____________________________________________________________________________________

Error Amplifier

A high-gain error amplifier provides accuracy for the

voltage-feedback loop regulation. Connect the neces-

sary compensation network between COMP and GND

(see the Compensation Design Guidelines section). The

error-amplifier transconductance is 1.5mS (typ). COMP

clamp low is set to 0.94V (typ), just below the slope ramp

compensation valley, helping COMP to rapidly return to

the correct 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

voltage transconductance value is 18A/V typ). To avoid

instability due to subharmonic oscillations when the duty

cycle is around 50% or higher, a slope compensation

ramp is added to the current-derived ramp waveform.

The compensation ramp slope (0.3V x 1MHz = 0.3V/Fs)

is equivalent to half the inductor current downslope in the

worst case (load 2A, current ripple 30% and maximum

duty-cycle operation of 94%). The slope compensation

ramp valley is set to 1.15V (typ).

Overcurrent Protection and Hiccup

When the converter output is shorted or the device is

overloaded, each high-side MOSFET current-limit event

(4A typ) turns off the high-side MOSFET and turns on the

low-side MOSFET. On each current-limit event a 3-bit

counter is incremented. The counter is reset after three

consecutive high-side MOSFETs turn on without reach-

ing current limit. If the current-limit condition persists,

the counter fills up reaching eight events. The control

logic then discharges SS/REFIN, stops both high-side

and low-side MOSFETs, and waits for a hiccup period

(1024 clock cycles typ) before attempting a new soft-

start sequence. The hiccup mode is also enabled during

soft-start time.

Thermal-Shutdown Protection

The MAX15053 contains an internal thermal sensor that

limits the total power dissipation to protect the device in

the event of an extended thermal fault condition. When

the die temperature exceeds +150NC (typ), the thermal

sensor shuts down the device, turning off the DC-DC

converter to allow the die to cool. After the die tempera-

ture falls by 20NC (typ), the device restarts, following the

soft-start sequence.

Skip Mode Operation

The MAX15053 operates in skip mode when SKIP is con-

nected to EN. When in skip mode, LX output becomes

high impedance when the inductor current falls below

200mA (typ). The inductor current does not become

negative. If during a clock cycle the inductor current falls

below the 200mA threshold (during off-time), the low side

turns off. At the next clock cycle, if the output voltage is

above set point, the PWM logic keeps both high-side

and low-side MOSFETs off. If instead the output voltage

is below the set point, the PWM logic drives the high-

side on for a minimum fixed on-time (300ns typ). In this

way the system can skip cycles, reducing frequency of

operations, and switches only as needed to service load

at the cost of an increase in output voltage ripple (see

the Skip Mode Frequency and Output Ripple section). In

skip mode, power dissipation is reduced and efficiency

is improved at light loads because power MOSFETs do

not switch at every clock cycle.

Applications Information

Setting the Output Voltage

The MAX15053 output voltage is adjustable from 0.6V

resistor-divider between the output and GND (Figure 1).

Choose R1 and R2 so that the DC errors due to the FB

input bias current (Q500nA) do not affect the output volt-

age accuracy. With lower value resistors, the DC error

is reduced, but the amount of power consumed in the

resistor-divider increases. A typical value for R2 is 10kI,

but values between 5kI and 50kI are acceptable. Once

R2 is chosen, calculate R1 using:

OUT

FB

V

R1 = R2

1

V





×

−









When regulating for an output of 0.6V in skip mode, short

FB to OUT and keep R2 connected from FB to GND.

Inductor Selection

A high-valued inductor results in reduced inductor ripple

current, leading to a reduced output ripple voltage.

However, a high-valued inductor results in either a larger

physical size or a high series resistance (DCR) and a

lower saturation current rating. Typically, choose an

inductor value to produce a current ripple equal to 30%

of load current. Choose the inductor with the following

formula:

OUT

OUT

SW

LOAD

IN

V

V

L

1

f

LIR I

V





=

× −





×

×





cy, and LIR is the desired inductor current ratio (typically