SI9138 Datasheet, PDF (13/14 Pages) Vishay Siliconix – Multi-Output, Individual On/Off Control Power-Supply Controller

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Si9138
Vishay Siliconix
DESCRIPTION OF OPERATION (CONT’D)
Buck Converter Operation:
The 3.3-V and 5-V buck converters are both current-mode
PWM and PSM (during light load operation) regulators using
high-side bootstrap n-channel and low-side n-channel
MOSFETs. At light load conditions, the converters switch at a
lower frequency than the clock frequency. This operating
condition is defined as pulse-skipping. The operation of the
converter(s) switching at clock frequency is defined as normal
operation.
Normal Operation: Buck Converters
In normal operation, the buck converter high-side MOSFET is
turned on with a delay (known as break-before-make time -
t_BBM), after the rising edge of the clock. After a certain on time,
the high-side MOSFET is turned off and then after a delay
(t_BBM), the low-side MOSFET is turned on until the next rising
edge of the clock, or the inductor current reaches zero. The
t_BBM (approximately 25 ns to 60 ns), has been optimized to
guarantee the efficiency is not adversely affected at the high
switching frequency and a specified minimum to account for
variations of possible MOSFET gate capacitances.
During the normal operation, the high-side MOSFET switch
on-time is controlled internally to provide excellent line and
load regulation over temperature. Both buck converters
should have load, line, regulation to within 0.5% tolerance.
Pulse Skipping: Buck Converters
When the buck converter switching frequency is less than the
internal clock frequency, its operation mode is defined as pulse
skipping mode. During this mode, the high-side MOSFET is
turned on until V_CS-V_FB reaches 20 mV, or the on time reaches
its maximum duty ratio. After the high-side MOSFET is turned
off, the low-side MOSFET is turned on after the t_BBM delay,
which will remain on until the inductor current reaches zero.
The output voltage will rise slightly above the regulation
voltage after this sequence, causing the controller to stay idle
for the next clock cycle, or several clock cycles. When the
output voltage falls slightly below the regulation level, the
high-side MOSFET will be turned on again at the next clock
cycle. With the converter remaining idle during some clock
cycles, the switching losses are reduced preserving
conversion efficiency during the light output current condition.
Current Limit: Buck Converters
When the buck converter inductor current is too high, the
voltage across pin CS3(5) and pin FB3(5) will exceed
Document Number: 71446
S-20642—Rev. B, 13-May-02
approximately 125 mV, causing the high-side MOSFET to be
turned off instantaneously regardless of the input, or output
condition. The Si9138 features clock cycle by clock cycle
current limiting capability.
Buck-Boost Converter Operation:
Designed mainly for PCMCIA or EEPROM programming, the
Si9138 has an adjustable 5-V to 12-V output non-isolated
buck-boost converter, called for brevity a Buck-Boost. The
input voltage range can span above or below the regulated
output voltage. It consists of two n-channel MOSFET switches
that are turned on and off in phase, and two diodes. Similar to
the buck converter, during the light load conditions, the
Buck-Boost converter will switch at a frequency lower than the
internal clock frequency, which can be defined as pulse
skipping mode (PSM); otherwise, it operates in normal PWM
mode.
The output voltage of the Buck-Boost converter is set by two
resistors (R₅ and R₆, see Figure 1) where,
V_BUBS
+
ǒ_R5
)
R₆
R₆^Ǔ
V_REF
Normal Operation: Buck-Boost Converter
In normal operation mode, the two MOSFETs are turned on at
the rising edge of the clock, and then turned off. The on time is
controlled internally to provide excellent load, line, and
temperature regulation. The Buck-Boost converter has load,
line and temperature regulation well within 0.5%.
Pulse Skipping: Buck-Boost Converter
Under the light load conditions, similar to the buck converter,
the Buck-Boost converter will enter pulse skipping mode. The
MOSFETs will be turned on until the inductor current increases
to such a level that the voltage across the pin CSP and pin CSN
reaches 410 mV, or the on time reaches the maximum duty
cycle. After the MOSFETs are turned off, the inductor current
will conduct through two diodes until it reaches zero. At this
point, the Buck-Boost converter output will rise slightly above
the regulation level, and the converter will stay idle for one or
several clock cycle(s) until the output falls back slightly below
the regulation level. The switching losses are reduced by
skipping pulses preserving the efficiency during light load.
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