lm25117psqx National Semiconductor Corporation, lm25117psqx Datasheet - Page 12

• Manufacturer Part Number: lm25117psqx
• Description: Wide Input Range Synchronous Buck Controller With Analog Current Monitor
• Manufacturer: National Semiconductor Corporation

www.national.com
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 re-
duces 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 ca-
pability 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 appli-
cations that need either a current monitor or current control.
The functional block diagram and typical application circuit of
the LM25117 are shown in
The device is available in TSSOP-20EP and LLP24 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 regu-
lator 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 capac-
itor connected to the VCC pin. The recommended capaci-
tance 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 temper-
ature exceeds the thermal shutdown threshold.
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
Figure 1
12
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 ex-
ternal bias supply is greater than the VIN pin voltage, an
external blocking diode is required from the input power sup-
ply to the VIN pin to prevent the external bias supply from
passing current to the input supply through VCC.
For V
directly to VCC through a diode.
For V
added to generate the external VCC supply voltage.
FIGURE 3. External VCC Supply for 5V< V
OUT
OUT
FIGURE 4. External VCC Supply for V
FIGURE 2. VIN Configuration for V
between 5V and 14.5V, the output can be connected
< 5V, a bias winding on the output inductor can be
VIN
OUT
<V
OUT
VCC
<5V
<14.5V
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