TPS56100
HIGHEFFICIENCY DSP POWER SUPPLY CONTROLLER
FOR 5V INPUT SYSTEMS
SLVS201A JUNE 1999 REVISED JULY 1999
6
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
detailed description (continued)
inhibit
INHIBIT is a TTL-compatible digital input used to enable the controller. When INHIBIT is low, the output drivers
are low and the slowstart capacitor is discharged. When INHIBIT goes high, the short across the slowstart
capacitor is released and normal converter operation begins. The 5-V supply must be above UVLO thresholds
before the controller is allowed to start up. The inhibit start threshold is 2.1 V and the hysteresis is 100 mV for
the INHIBIT comparator.
VCC undervoltage lockout (UVLO)
The undervoltage lockout circuit disables the controller while the VCC supply is below the 4-V start threshold
during power up. When the controller is disabled, the output drivers will be low and the slowstart capacitor is
discharged. When VCC exceeds the start threshold, the short across the slowstart capacitor is released and
normal converter operation begins. There is a 0.5-V hysteresis in the undervoltage lockout circuit for noise
immunity.
slowstart
The slowstart circuit controls the rate at which VO powers up. A capacitor is connected between SLOWST and
ANAGND and is charged by an internal current source. The current source is proportional to the reference
voltage, so that the charging rate of CSLOWST is proportional to the reference voltage. By making the charging
current proportional to VREF, the power-up time for VO will be independent of VREF. Thus, CSLOWST can remain
the same value for all VP settings. The slowstart charging current is determined by the following equation:
Islowstart = I(VREFB) / 5 (amps)
Where I(VREFB) is the current flowing out of VREFB.
It is recommended that no additional loads be connected to VREFB, other than the resistor divider for setting the
hysteresis voltage. The maximum current that can be sourced by the VREFB circuit is 500 A. The equation for
setting the slowstart time is:
tSLOWST = 5 × CSLOWST × RVREFB
(seconds)
Where RVREFB is the total external resistance from VREFB to ANAGND.
power good
The power-good circuit monitors for an undervoltage condition on VO. If VO is 7% below VREF, then the PWRGD
pin is pulled low. PWRGD is an open-drain output.
overvoltage protection
The overvoltage protection (OVP) circuit monitors VO for an overvoltage condition. If VO is 15% above VREF,
then a fault latch is set and both output drivers are turned off. The latch will remain set until VCC goes below the
undervoltage lockout value or INHIBIT is low. A 3-
s deglitch timer is included for noise immunity. Refer to the
LODRV section for information on how to protect the microprocessor against overvoltages due to a shorted
high-side power FET.