LM25007MMX Datasheet, PDF (10/19 Pages) Texas Instruments – LM25007 42V, 0.5A Step-Down Switching Regulator

LM25007
SNVS401C – JANUARY 2006 – REVISED FEBRUARY 2013
V_IN
VIN
7V SERIES
REGULATOR
www.ti.com
R_ON
RON/SD
STOP
RUN
RON
ON TIMER
VIN
START
COMPLETE
LM25007
Figure 8. Shutdown Implementation
Current Limit
The LM25007 contains an intelligent current limit off-timer intended to reduce the foldback characteristic inherent
with fixed off-time over-current protection. If the current in the Buck switch exceeds 725mA the present cycle on-
time is immediately terminated (cycle by cycle current limit). Following the termination of the cycle a non-
resetable current limit off timer is initiated. The duration of the off time is a function of the external resistor (RCL)
and the FB pin voltage. When the FB pin voltage equals zero, the current limit off-time is internally preset to
17µs. This condition occurs in short circuit operation when a maximum amount of off-time is required.
In cases of overload (not complete short circuit) the current limit off-time is reduced as a function of the output
voltage (measured at the FB pin). Reducing the off-time with smaller overloads reduces the amount of foldback
and also reduces the initial start-up time. The current limit off time for a given FB pin voltage and R_CL resistor can
be calculated by the following equation:
Toff = 10^-5 / (0.59 + (V_FB / 7.22 x 10^-6 x R_CL))
(5)
Applications utilizing low resistance inductors and/or a low voltage drop rectifier may require special evaluation at
high line, short circuit conditions. In this special case the preset 17µs (FB = 0V) off-time may be insufficient to
balance the inductor volt*time product. Additional inductor resistance, output resistance or a larger voltage drop
rectifier may be necessary to balance the inductor cycle volt*time product and limit the short circuit current.
N - Channel Buck Switch and Driver
The LM25007 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. The
bootstrap capacitor is charged by V_CC through the internal high voltage diode. A 0.01uF ceramic capacitor
connected between the BST pin and SW pin is recommended.
During each cycle when the Buck switch turns off, the SW pin is approximately 0V. When the SW pin voltage is
low, the bootstrap capacitor is charged from Vcc through the internal diode. The minimum off timer, set to 300ns,
ensures that there will be a minimum interval every cycle to recharge the bootstrap capacitor.
An external re-circulating diode from the SW pin to ground is necessary to carry the inductor current after the
internal Buck switch turns off. This external diode must be of the Ultra-fast or Schottky type to reduce turn-on
losses and current over-shoot. The reverse voltage rating of the re-circulating diode must be greater than the
maximum line input voltage.
10
Submit Documentation Feedback
Product Folder Links: LM25007
Copyright © 2006–2013, Texas Instruments Incorporated