参数资料
| 型号： | ISL6237IRZ-T |
| 厂商： | Intersil |
| 文件页数： | 26/35页 |
| 文件大小： | 0K |
| 描述： | IC MAIN PWR CTRLR QUAD 32-QFN |
| 产品培训模块： | Solutions for Industrial Control Applications |
| 标准包装： | 6,000 |
| 应用： | 控制器，笔记本电脑电源系统 |
| 输入电压： | 5.5 V ~ 25 V |
| 输出数： | 4 |
| 输出电压： | 多重 |
| 工作温度： | -40°C ~ 100°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-VFQFN 裸露焊盘 |
| 供应商设备封装： | 32-QFN 裸露焊盘（5x5） |
| 包装： | 带卷 (TR) |
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ISL6237
K ? V OUT V IN – V OUT
I LOAD ( SKIP ) = ------------------------ --------------------------------
2 ? L
Automatic Pulse-Skipping Switchover
(Idle Mode)
In Idle 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. This
mechanism causes the threshold between pulse-skipping
PFM and non skipping PWM operation to coincide with the
boundary between continuous and discontinuous
inductor-current operation (also known as the critical
conduction point):
(EQ. 3)
V IN
PFM/PWM crossover occurs, I LOAD(SKIP) , is equal to half
the peak-to-peak ripple current, which is a function of the
inductor value (Figure 67). For example, in the ISL6237
typical application circuit with V OUT1 = 5V, V IN = 12V,
L = 7.6μH, and K = 5μs, switchover to pulse-skipping
operation occurs at I LOAD = 0.96A or about on-fifth full load.
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used.
The switching waveforms may appear noisy and
asynchronous when light loading causes pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM noise
vs light-load efficiency are made by varying the inductor
value. Generally, low inductor values produce a broader
efficiency vs load curve, while higher values result in higher
full-load efficiency (assuming that the coil resistance remains
fixed) and less output voltage ripple. Penalties for using
higher inductor values include larger physical size and
degraded load-transient response (especially at low
input-voltage levels).
DC output accuracy specifications refer to the trip level of the
error comparator. When the inductor is in continuous
conduction, the output voltage has a DC regulation higher
than the trip level by 50% of the ripple. In discontinuous
conduction (SKIP = GND, light load), the output voltage has
a DC regulation higher than the trip level by approximately
1.0% due to slope compensation.
Forced-PWM Mode
The low-noise, forced-PWM (SKIP = VCC) mode disables
the zero-crossing comparator, which controls the low-side
switch on-time. Disabling the zero-crossing detector causes
DI
t
=
V IN -V OUT
L
I PEAK
the low-side, gate-drive waveform to become the
complement of the high-side, gate-drive waveform. The
inductor current reverses at light loads as the PWM loop
strives to maintain a duty ratio of V OUT /V IN . The benefit of
forced-PWM mode is to keep the switching frequency fairly
constant, but it comes at a cost: the no-load battery current
I LOAD = I PEAK/2
can be 10mA to 50mA, depending on switching frequency
and the external MOSFETs.
Forced-PWM mode is most useful for reducing
audio-frequency noise, improving load-transient response,
providing sink-current capability for dynamic output voltage
adjustment, and improving the cross-regulation of
0
ON-TIME
TIME
multiple-output applications that use a flyback transformer or
FIGURE 67. ULTRASONIC CURRENT WAVEFORMS
40μs (MAX)
INDUCTOR
CURRENT
Zero-Crossing
ZERO-CROSSING
DETECTION
0A
FB < REG.POINT
ON-TIME (t ON )
FIGURE 68. ULTRASONIC CURRENT WAVEFORMS
26
coupled inductor.
Enhanced Ultrasonic Mode
(25kHz (min) Pulse Skipping)
Leaving SKIP unconnected or connecting SKIP to REF
activates a unique pulse-skipping mode with a minimum
switching frequency of 25kHz. 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)).
An ultrasonic pulse occurs when the controller detects that
no switching has occurred within the last 20μs. Once
triggered, the ultrasonic controller pulls LGATE high, turning
on the low-side MOSFET to induce a negative inductor
FN6418.4
March 18, 2008
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