THMC40 Datasheet, PDF (9/17 Pages) Texas Instruments – VARIABLE SPEED 12-VDC BRUSHLESS FAN MOTOR DRIVERS

THMC40, THMC41
VARIABLE SPEED 12-VDC BRUSHLESS FAN MOTOR DRIVERS
SLIS097 – MARCH 2000
charge pump (CP)
THMC40 and THMC41 have an internal charge pump which utilizes an external reservoir capacitor (CP) to
generate the gate-drive voltage for the V_OUT output high-side DMOS transistor (see Figure 3). The
recommended value for CP is 0.1 µF, with a minimum rating of 35 WVdc. The charge pump is disabled during
the sleep state (see the sleep/run state section) to minimize current consumption into V_PWR. The charge pump
also incorporates internal undervoltage lockout detection used to disable V_OUT when the charge pump voltage
does not have an adequate level above V_PWR to fully drive the high-side DMOS gate. Thus, the V_CP (UVLO),
in conjunction with the charge pump start-up time, delays the V_OUT drive for a short time after the device has
transitioned from sleep to run state, until the charge pump voltage reaches the UVLO threshold.
PWM oscillator/triangle waveform generator (C_OSC)
The PWM oscillator uses source and sink currents switched into an external capacitor (C_OSC) to set the PWM
frequency and generate a triangle waveform. A PWM oscillator cycle consists of charging C_OSC with a constant
current source (–180 µA typical) until the C_OSC voltage ramps up to an upper threshold (2.3 V typical), and then
discharging C_OSC with a constant current sink (180 µA typical) until the C_OSC voltage ramps down to a lower
threshold (0.5 V typical). The charge/discharge cycle is repeated each time the 2.3-V or 0.5-V threshold is
reached (see Figure 5).
VPWR
VREF
–180 µA
COSC
2.3 V
2200 pF
0.5 V
180 µA
S
Q
R
Q
VPWM
PWM
V
2.3 V
VCOSC
0.5 V
Q
Q
0
Time
Figure 5. PWM Triangle Waveform Generator
The following equation can be used to calculate the value of C_OSC needed for a desired PWM frequency, f_PWM:
+ C_{OSC(max) 2}
ǒ _* Ǔ ^ICHARGE(min)
f_PWM(min) V_DISCHARGE
V_CHARGE
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