SH3100IMTR SEMTECH [Semtech Corporation], SH3100IMTR Datasheet - Page 66

• Manufacturer Part Number: SH3100IMTR
• Description: Supervisory IC with I2C Interface and PWM
• Manufacturer: SEMTECH [Semtech Corporation]

PWM Operation & Fan Speed Control
The PWM function is implemented as a 10-bit PWM coun-
ter clocked at the CLK1 rate. Therefore, if CLK1 is set to
32.768 kHz, the PWM repeat rate is 32 Hz.
The PWM duty cycle can either be set directly by writing to
the ForcePWMDutyCycle register or may be programmed
to track the temperature in AutoPWMMode .
The PWM function normally operates in standard PWM
output format, but it can be programmed to operate in
PDM (Pulse Density Modulation) mode, which outputs the
same energy density, but distributed more evenly over the
PWM cycle. This is useful when the PDM output is going
to be fi ltered to generate a DC level.
Auto-PWM Mode (Fan Control Mode)
This is an application of the PWM function which oper-
ates in conjunction with the general purpose 8-bit DAC
and comparator to provide automatic fan speed control.
It can be enabled by setting the SMPS mode bits in the
registers such that operation commences immediately on
power-up. The external application circuitry is shown in
operating mode 3.
AutoPWMMode has been designed as a fl exible fan con-
trol engine for brushless DC motors, providing a PWM duty
cycle proportional to the temperature. When in AutoPWM-
Mode , temperature is measured once per 32 PWM cycles
which means once per second at the power-up default
rate of 32 Hz.
Fan speed control uses the SNSE input to detect when
the fan has stalled. While the fan is running, voltage tran-
sients on the SNSE pin crossing the set threshold indicate
to the controller that the fan is running correctly. If the
SNSE transitions cease for more than 1 second, then this
indicates that the fan has stalled, and the PWM duty cycle
should be set to 100% to try to restart the fan. Both posi-
tive and negative going transitions are detected, so if the
SNSE signal stops in any state, then fan stall is detected.
© 2006 Semtech Corp.
POWER MANAGEMENT
Functional Descriptions
(continued)
66
Auto-PWM Engine Features:
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7-bit minimum temperature setting register, which de-
fi nes, with 2°C resolution, the temperature at which
the PWM comes on. The state machine incorporates
4°C of hysteresis to prevent the fan from continually
switching on & off when the temperature is approxi-
mately the minimum.
10-bit minimum duty cycle register (shared with the
ForcePWMDuty when not in AutoPWMMode ), which
defi nes the starting duty cycle.
5-bit DutyCycleStepSize register, which determines
the duty cycle increments per °C. This register along
with the MinTemp register determines the maximum
temperature.
The PWM starts at 100% duty cycle for between 1 and
2 seconds to kick-start the motor, before settling to
MinDutyCycle. Thereafter it tracks with temperature.
If the SNSE input pulses cease, the PWM generates
100% duty cycle for 1 or 2 seconds, and if there are
still no pulses, enters the FAULT state and generates
an interrupt.
If the duty cycle reaches 100% due to the maximum
temperature being reached, an interrupt is generat-
ed.
Software can read the AutoPWMDutyCycle register to
determine the current duty cycle is for diagnostic pur-
poses.
The PWM counter is clocked from the same source as
CLK1. The CLK1 output does not need to be enabled.
In theory, this allows the PWM rate to be varied up
to a maximum PWM repeat rate of 32.7 kHz (33.5
MHz/1024) , but since the temperature measure-
ment interval scales with PWM rate, the ADC conver-
sion rate or the SNSE pulse repeat rate limits the ef-
fective maximum CLK1 frequency.
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SH3100