Datasheet

Drivers Off
UVLO
Disable
UVLO
7.6V/6.2V
THERMAL
SHUTDOWN
Max
.
Duty
Cycle Limiter
20PA
LM5034
V
PWR
V
CC
V
CC
1.25V
R
1
R
2
LM5034
www.ti.com SNVS347A –FEBRUARY 2005–REVISED APRIL 2013
Figure 20. Drivers Off and V
CC
Disable
Startup Regulator, VIN, VCC1, VCC2
The high voltage startup regulator is integral to the LM5034. The input pin VIN can be connected directly to a
voltage between 13V and 100V, with transient capability to 105V. The startup regulator provides bias voltages to
the series pass V
CC
regulator and the UVLO circuit. The V
CC
regulator is disabled until the voltage at the UVLO
pin (described above) exceeds 1.25V. For applications where V
PWR
exceeds 100V the internal startup regulator
can be powered from an external startup regulator or other available low voltage source. See Applications
Information for details.
The V
CC
under-voltage threshold circuit (UVT) monitors the VCC regulator output. When the series pass
regulator is enabled and the internal V
CC
voltage increases to > 7.6V, the UVT comparator activates the PWM
controller and output drivers via the Drivers Off signal. The UVT comparator has built-in hysteresis, with the lower
threshold nominally set to 6.2V. See Figure 5 and Figure 20.
When enabled, the V
CC
regulated output is 7.7V ±4% with current limited to a minimum of 19 mA (typically 22
mA). The regulator’s output is split by a resistor divider to provide separate VCC1 and VCC2 rails for the two
controller channels. VCC1 powers Controller 1, drivers OUT1 and AC1, Soft-start1, and all the support functions.
VCC2 powers Controller 2, drivers OUT2 and AC2, and Soft-start2. If VCC1 and/or VCC2 are used to power
external circuitry, the current limit specification applies to the sum of the load currents at the two pins.
Splitting the V
CC
regulator output through two 5Ω resistors allows separate external V
CC
bypass capacitors to
reduce cross-talk between channels. Each VCC output pin requires a capacitor to its corresponding ground for
stability, as well as to provide the surge currents to the external MOSFETs via the gate driver outputs. The
capacitors should be the same value, and be physically close to their respective pins.
In most applications it is necessary to power V
CC
from an external source as the average current required at the
output drivers may exceed the current capability of the internal regulator and/or the thermal capability of the
LM5034 package (see Figure 8). Normally the external source is derived from the converter’s power stage once
the LM5034 outputs are active. See Applications Information for more information.
Drivers Off, V
CC
Disable
Referring to Figure 20, Drivers Off and V
CC
Disable are internal signals which, when active disable portions of the
LM5034. If the UVLO pin is below 1.25V, or if the thermal shutdown activates, the V
CC
Disable line switches high
to disable the V
CC
regulator. UVLO also activates the Drivers Off signal to disable the output drivers, connect the
SS1, SS2, COMP1, COMP2 and RES pins to ground, and enable the 50 µA Soft-start current sources.
If the V
CC
voltage falls below the under-voltage threshold of 6.2V , the UVT comparator activates only the Drivers
Off signal. The output drivers are disabled but the V
CC
regulator is not disabled. Additionally, the CS1, CS2, SS1,
SS2, COMP1, COMP2 and RES pins are internally grounded, and the 50 µA Soft-start current sources are
enabled.
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