Datasheet

VIN
LM25117
V
IN
VCC
External
VCC Supply
LM25117
LM25117-Q1
www.ti.com SNVS714E –APRIL 2011–REVISED MARCH 2013
FUNCTIONAL DESCRIPTION
The LM25117 high voltage switching controller features all of the functions necessary to implement an efficient
high voltage buck regulator that operates over a very wide input voltage range. This easy to use controller
integrates high-side and low-side NMOS drivers. The regulator control method is based upon peak current mode
control utilizing an emulated current ramp. Peak current mode control provides inherent line feed-forward, cycle-
by-cycle current limiting and ease of loop compensation. The use of an emulated control ramp reduces noise
sensitivity of the PWM circuit, allowing reliable processing of the very small duty cycles necessary in high input
voltage applications.
The switching frequency is user programmable up to 750 kHz. The RT pin allows the switching frequency to be
programmed by a single resistor or synchronized to an external clock. Fault protection features include cycle-by-
cycle and hiccup mode current limiting, thermal shutdown and remote shutdown capability by pulling down UVLO
pin. The UVLO input enables the regulator when the input voltage reaches a user selected threshold and
provides a very low quiescent shutdown current when pulled low. A unique analog telemetry feature provides
averaged output current information, allowing various applications that need either a current monitor or current
control. The functional block diagram and typical application circuit of the LM25117 are shown in Figure 17
The device is available in HTSSOP-20 and WQFN-24 (4mm×4mm) package featuring an exposed pad to aid in
thermal dissipation.
High Voltage Startup Regulator and VCC Disable
The LM25117 contains an internal high voltage bias regulator that provides the VCC bias supply for the PWM
controller and NMOS gate drivers. The VIN pin can be connected to an input voltage source as high as 42V. The
output of the VCC regulator is set to 7.6V. When the input voltage is below the VCC set-point level, the VCC
output tracks the VIN with a small dropout voltage. The output of the VCC regulator is current limited at 30mA
minimum.
Upon power-up, the regulator sources current into the capacitor connected to the VCC pin. The recommended
capacitance range for the pin VCC is 0.47µF to 10µF. When the VCC pin voltage exceeds the VCC UV threshold
and the UVLO pin is greater than UVLO threshold, the HO and LO drivers are enabled and a soft-start sequence
begins. The HO and LO drivers remain enabled until either the VCC pin voltage falls below VCC UV threshold,
the UVLO pin voltage falls below UVLO threshold, hiccup mode is activated or the die temperature exceeds the
thermal shutdown threshold. Enabling/Disabling the IC by controlling UVLO is recommended in most of cases.
An output voltage derived bias supply can be applied to the VCC pin to reduce the controller power dissipation at
higher input voltage. The VCCDIS input can be used to disable the internal VCC regulator when external biasing
is supplied. The externally supplied bias should be coupled to the VCC pin through a diode, preferably a
Schottky diode. If the VCCDIS pin voltage exceeds the VCCDIS threshold, the internal VCC regulator is disabled.
VCCDIS has a 500kΩ internal pull-down resistor to ground for normal operation with no external bias.
The VCC regulator series pass transistor includes a diode between VCC (Anode) and VIN (Cathode) that should
not be forward biased in normal operation. If the voltage of the external bias supply is greater than the VIN pin
voltage, an external blocking diode is required from the input power supply to the VIN pin to prevent the external
bias supply from passing current to the input supply through VCC.
Figure 18. VIN Configuration for V
VIN
<V
VCC
For V
OUT
between 5V and 14.5V, the output can be connected directly to VCC through a diode.
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Product Folder Links: LM25117 LM25117-Q1