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| 型号： | MAX17101ETJ+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 21/31页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR DIV PWM CM 32TQFNEP |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 3 |
| 频率 - 最大： | 500kHz |
| 电源电压： | 6 V ~ 24 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 是 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | 0°C ~ 85°C |
| 封装/外壳： | 32-WFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
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Dual Quick-PWM, Step-Down Controller
with Low-Power LDO, RTC Regulator
Modes of Operation
Forced-PWM Mode ( S K I P = V CC )
The low-noise forced-PWM mode ( SKIP = V CC ) 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 V OUT /V IN . 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 ( S K I P = 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 I OUT < I LOAD(SKIP) ), the integrator cannot
correct for the low-frequency output ripple error, so the
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:
V NEG(US) = I L R CS = (V NOM - V FB ) x 0.385V
where V NOM is the nominal feedback-regulation volt-
age, and V FB is the actual feedback voltage (V FB >
V NOM ), and R CS is the current-sense resistance seen
across LX to AGND.
37 μ s (typ)
INDUCTOR
CURRENT
ZERO-CROSSING
DETECTION
0
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 ( S K I P = Open or REF)
I SONIC
ON-TIME (t ON )
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
(I LOAD(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
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.
______________________________________________________________________________________
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