800mA, 2MHz, PWM DC-to-DC

8

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MANUFACTURER

PART

VALUE (µH)

SIZE (mm)

SHIELDED

Murata

LQH32CN

2.2

97

790

2.5 x 3.2 x 2.0

No

CDRH3D16

2.2

50

1200

3.8 x 3.8 x 1.8

Yes

Sumida

CDRH2D11

2.2

78

780

3.2 x 3.2 x 1.2

Yes

D312F

2.2

170

1200

3.6 x 3.6 x 1.2

No

TOKO

D412F

2.2

140

1330

4.8 x 4.8 x 1.2

No

Table 2. Recommended Inductors

output and storing energy in the inductor’s magnetic

field. The current-mode feedback system regulates the

peak inductor current as a function of the output voltage

error signal. Since the average inductor current is nearly

the same as the peak inductor current (assuming that

the inductor value is relatively high to minimize ripple

current), the circuit acts as a switch-mode transconduc-

tance amplifier. This pushes the output LC filter pole,

normally found in a voltage-mode PWM, to a higher fre-

quency. To preserve inner-loop stability and eliminate

inductor staircasing, an internal slope-compensation

ramp is summed into the main PWM comparator. During

the second half of the switching cycle (off-time), the

internal high-side P-channel MOSFET turns off and the

internal low-side N-channel MOSFET turns on. Now the

inductor releases the stored energy as its current ramps

down while still providing current to the output. The output

capacitor stores charge when the inductor current

exceeds the load current and discharges when the

inductor current is lower, smoothing the voltage across

the load. Under overload conditions, when the inductor

current exceeds the current limit, the high-side MOSFET

is turned off and the low-side MOSFET remains on for

the remainder of the cycle to let the inductor current

ramp down.

Pulse-Group Mode

Pulse-group mode is used to minimize the supply cur-

rent with a light load. In pulse-group mode, the IC shuts

the nominal regulation voltage, the IC powers up its cir-

cuits and resumes switching.

Pulse-Skip Mode

Pulse-skip mode is also used to minimize the supply

current with a light load. The difference between pulse-

above the +0.8% regulation point, pulse-group mode

stops switching and completely turns off a number of

circuits. Under the same conditions, pulse-skip mode

stops switching but leaves all circuits on. The delay

coming out of pulse-skip mode is shorter than with

pulse-group mode. In pulse-skip mode, the output volt-

age ripple is lower, and the load-transient response

faster. However, the quiescent current is higher than in

pulse-group mode.

Forced-PWM Mode

In forced-PWM mode, the MAX1572 operates at a con-

stant 2MHz switching frequency without pulse skipping.

This is desirable in noise-sensitive applications, since the

output ripple is minimized and has a predictable noise

spectrum. Forced-PWM mode requires higher supply

current with light loads due to constant switching.

100% Duty-Cycle Operation

The MAX1572 can operate at 100% duty cycle. In this

state, the high-side P-channel MOSFET is turned on (not

switching). This occurs when the input voltage is close to

the output voltage. The dropout voltage is the voltage

drop due to the output current across the on-resistance

Table 2.

Load-Transient Response/

Voltage Positioning

The MAX1572 uses voltage positioning that matches

the load regulation to the voltage droop seen during

load transients. In this way, the output voltage does not

overshoot when the load is removed, which results in

the total output-voltage variation being half as wide as

in a conventional design. Figure 2 shows an example of

a voltage-positioned and a nonvoltage-positioned load

transient. Additionally, the MAX1572 uses a wide-band-

width feedback loop to respond more quickly to a load

transient than regulators using conventional integrating

feedback loops.

The load line used to achieve voltage positioning is

shown in Figure 3. This assumes a nominal operating

point of 3.6V input at 300mA load.