MAX17101 查看數據表（PDF） - Maxim Integrated

零件编号

产品描述 (功能)

生产厂家

MAX17101

Dual Quick-PWM, Step-Down Controller with Low-Power LDO, RTC Regulator

Maxim Integrated 

Dual Quick-PWM, Step-Down Controller

with Low-Power LDO, RTC Regulator

Modes of Operation

Forced-PWM Mode (SKIP = VCC)

The low-noise forced-PWM mode (SKIP = VCC) dis-

ables the zero-crossing comparator, which controls the

low-side switch on-time. This forces the low-side gate-

drive waveform to constantly be the complement of the

high-side gate-drive waveform, so the inductor current

reverses at light loads while DH maintains a duty factor

of VOUT/VIN. The benefit of forced-PWM mode is to

keep the switching frequency fairly constant. However,

forced-PWM operation comes at a cost: the no-load 5V

bias current remains between 20mA to 60mA depend-

ing on the switching frequency and MOSFET selection.

The MAX17101 automatically uses forced-PWM opera-

tion during all transitions—startup and shutdown—

regardless of the SKIP configuration.

Automatic Pulse-Skipping Mode (SKIP = GND)

In skip mode (SKIP = GND), an inherent automatic

switchover to PFM takes place at light loads. This

switchover is affected by a comparator that truncates

the low-side switch on-time at the inductor current’s

zero crossing. The zero-crossing comparator threshold

is set by the differential across LX and AGND.

DC output-accuracy specifications refer to the integrat-

ed threshold of the error comparator. When the induc-

tor is in continuous conduction, the MAX17101

regulates the valley of the output ripple and the internal

integrator removes the actual DC output-voltage error

caused by the output-ripple voltage and internal slope

compensation. In discontinuous conduction (SKIP =

GND and IOUT < ILOAD(SKIP)), the integrator cannot

correct for the low-frequency output ripple error, so the

output voltage has a DC regulation level higher than the

error comparator threshold by approximately 1.5% due

to slope compensation and output ripple voltage.

Ultrasonic Mode (SKIP = Open or REF)

Leaving SKIP unconnected or connecting SKIP to REF

(2V) activates a unique pulse-skipping mode with a

guaranteed minimum switching frequency of 20kHz.

This ultrasonic pulse-skipping mode eliminates audio-

frequency modulation that would otherwise be present

when a lightly loaded controller automatically skips

pulses. In ultrasonic mode, the controller automatically

transitions to fixed-frequency PWM operation when the

load reaches the same critical conduction point

(ILOAD(SKIP)) that occurs when normally pulse skipping.

An ultrasonic pulse occurs (Figure 4) when the con-

troller detects that no switching has occurred within the

last 37μs or when SECFB drops below its feedback

threshold. Once triggered, the ultrasonic circuitry pulls

DL high, turning on the low-side MOSFET to induce a

negative inductor current. After the inductor current

reaches the negative ultrasonic current threshold, the

controller turns off the low-side MOFET (DL pulled low)

and triggers a constant on-time (DH driven high). When

the on-time has expired, the controller reenables the

low-side MOSFET until the inductor current drops

below the zero-crossing threshold. Starting with a DL

pulse greatly reduces the peak output voltage when

compared to starting with a DH pulse.

The output voltage at the beginning of the ultrasonic

pulse determines the negative ultrasonic current

threshold, resulting in the following equation:

VNEG(US) = ILRCS = (VNOM - VFB) x 0.385V

where VNOM is the nominal feedback-regulation volt-

age, and VFB is the actual feedback voltage (VFB >

VNOM), and RCS is the current-sense resistance seen

across LX to AGND.

37μs (typ)

INDUCTOR

CURRENT

0

ISONIC

ZERO-CROSSING

DETECTION

ON-TIME (tON)

Figure 4. Ultrasonic Waveforms

Secondary Feedback: SECFB—OUT1 ONLY

When the controller skips pulses (SKIP = GND or REF),

the long time between pulses (especially if the output is

sinking current) allows the external charge-pump voltage

or transformer secondary winding voltage to drop. When

the SECFB voltage drops below its 2V feedback thresh-

old, the MAX17101 issues an ultrasonic pulse (regardless

of the ultrasonic one-shot state). This forces a switching

cycle, allowing the external unregulated charge pump (or

transformer secondary winding) to be refreshed. See the

Ultrasonic Mode (SKIP = Open or REF) section for

switching cycle sequence/specifications.

______________________________________________________________________________________ 21

Share Link: