LM34922 Datasheet, PDF (12/17 Pages) Texas Instruments – 28V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit

FIGURE 2. Normal and Current Limit Operation
30140724
Ripple Requirements
The LM34922 requires a minimum of 15mVp-p ripple voltage
at the CS pin. That ripple voltage is generated by the de-
creasing recirculating current (the inductor’s ripple current)
through R_S during the off-time. See Figure 3a.
CS Pin Waveform
30140725
FIGURE 3.
The ripple voltage is equal to:
V_RIPPLE = ΔI x R_S
where ΔI is the inductor current ripple amplitude, and R_S is
the current sense resistor at the CS pin.
N-Channel Buck Switch and Driver
The LM34922 integrates an N-Channel buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capac-
itor (C_BST) and an internal high voltage diode. A 0.1µF capac-
itor connected between BST and SW provides the supply
voltage for the driver during the on-time. During each off-time,
the SW pin is at approximately -1V, and C_BST is recharged
from the internal 5V regulator for the next on-time. The mini-
mum off-time ensures a sufficient time each cycle to recharge
the bootstrap capacitor.
Soft-Start
The soft-start feature allows the converter to gradually reach
a steady state operating point, thereby reducing startup
stresses and current surges. Upon turn-on, when V_IN reaches
its under-voltage lock-out threshold an internal 10µA current
source charges the external capacitor at the SS pin to 2.51V
(t1 in Figure 1). The ramping voltage at SS ramps the non-
inverting input of the regulation comparator, and the output
voltage, in a controlled manner. For proper operation, the soft-
start capacitor should be no smaller than 1000pF.
The LM34922 can be employed as a tracking regulator by
applying the controlling voltage to the SS pin. The regulator’s
output voltage tracks the applied voltage, gained up by the
ratio of the feedback resistors. The applied voltage at the SS
pin must be within the range of 0.5V to 2.6V. The absolute
maximum rating for the SS pin is 3.0V. If the tracking function
causes the voltage at the FB pin to go below the thresholds
for the PGD pin, the PGD pin will switch low (see the Power
Good Output section). An internal switch grounds the SS pin
if the input voltage at VIN is below its under-voltage lock-out
threshold or if the Thermal Shutdown activates. If the tracking
function (described above) is used, the tracking voltage ap-
plied to the SS pin must be current limited to a maximum of
1mA.
www.ti.com
12