LM34919C-Q1 Datasheet, PDF (15/29 Pages) Texas Instruments – LM34919C-Q1 Ultra Small 50V, 600 mA Constant On-Time Buck Switching Regulator

LM34919C-Q1
www.ti.com
SNVS831A – SEPTEMBER 2013 – REVISED DECEMBER 2013
Current Limit
Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling
diode (D1). Referring to Figure 5, when the buck switch is turned off the inductor current flows out of ISEN and
through D1. If the valley point of that current exceeds 0.64 A the current limit comparator output switches to
delay the start of the next on-time period. The next on-time starts when the valley point of the current out of ISEN
is below 0.64 A and the voltage at FB is below 2.52 V. If the overload condition persists causing the inductor
current valley point to exceed 0.64 A during each cycle the operating frequency is lower due to longer-than-
normal off-times.
Figure 8 illustrates the inductor current waveform. During normal operation the load current is Io, the average of
the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches
0.64 A. During the Current Limited portion of Figure 8, the current ramps down to 0.64 A during each off-time,
initiating the next on-time (assuming the voltage at FB is <2.52 V). During each on-time the current ramps up an
amount equal to:
ΔI = (V_IN - V_OUT) x t_ON / L1
(6)
During this time the LM34919C operates in a constant current mode with an average load current (I_OCL) equal to
0.64 A + ΔI/2.
Generally, in applications where the switching frequency is higher than ≊300 kHz and a relatively small value
inductor is used, the higher dl/dt of the inductor's ripple current results in an effectively lower valley current limit
threshold due to the response time of the current limit detection circuit. However, since the small value inductor
results in a relatively high ripple current amplitude (ΔI in Figure 8), the load current (I_OCL) at current limit typically
exceeds 640 mA.
I_PK
I
I_OCL
0.64A
I_O
Normal Operation
Load Current
Increases
Current Limited
Figure 8. Inductor Current - Current Limit Operation
N - Channel Buck Switch and Driver
The LM34919C integrates an N-Channel buck switch and associated floating high side gate driver. The peak
current allowed through the buck switch is 1.5 A, and the maximum allowed average current is 1 A. The gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022
µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During
each off-time, the SW pin is at approximately -1 V, and C4 charges from V_CC through the internal diode. The
minimum off-time of LM34919C ensures 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
start-up stresses and current surges. Upon turn-on, after V_CC reaches the undervoltage threshold, an internal
10.5 µA current source charges up the external capacitor at the SS pin to 2.52 V. The ramping voltage at SS
(and the inverting input of the regulation comparator) ramps up the output voltage in a controlled manner.
An internal switch grounds the SS pin if V_CC is below the undervoltage lockout threshold, or if the EN pin is
grounded.
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