LM34922_15 Datasheet, PDF (9/19 Pages) Texas Instruments – 28V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit

LM34922
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FUNCTIONAL DESCRIPTION
SNVS813A – JUNE 2012 – REVISED MARCH 2013
The LM34922 Constant On-time Step-down Switching Regulator features all the functions needed to implement a
low cost, efficient buck bias power converter capable of supplying up to 2.0A to the load. This high voltage
regulator contains an N-Channel buck switch, is easy to implement, and is available in a 10-pin HVSSOP-
PowerPAD power enhanced package. The regulator’s operation is based on a constant on-time control principle
with the on-time inversely proportional to the input voltage. This feature results in the operating frequency
remaining relatively constant with load and input voltage variations. The constant on-time feedback control
principle requires no loop compensation resulting in very fast load transient response. The adjustable valley
current limit detection results in a smooth transition from constant voltage to constant current when current limit is
reached. To aid in controlling excessive switch current due to a possible saturating inductor the on-time is
reduced by ≊40% when current limit is detected. The Power Good output (PGD pin) indicates when the output
voltage is within 5% of the expected regulation voltage.
The LM34922 can be implemented to efficiently step-down higher voltages in non-isolated applications.
Additional features include: Low output ripple, VIN under-voltage lock-out, adjustable soft-start timing, thermal
shutdown, gate drive pre-charge, gate drive under-voltage lock-out, and maximum duty cycle limit.
Control Circuit Overview
The LM34922 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with
the output voltage feedback (FB) compared to an internal reference (2.51V). If the FB voltage is below the
reference the internal buck switch is switched on for the one-shot timer period, which is a function of the input
voltage and the programming resistor (R_T). Following the on-time the switch remains off until the FB voltage falls
below the reference, but never less than the minimum off-time forced by the off-time one-shot timer. When the
FB pin voltage falls below the reference and the off-time one-shot period expires, the buck switch is then turned
on for another on-time one-shot period.
When in regulation, the LM34922 operates in continuous conduction mode at heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode the inductor’s current is
always greater than zero, and the operating frequency remains relatively constant with load and line variations.
The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude.
The approximate operating frequency is calculated as follows:
V_OUT
F_S = _{(4.1 x 10-11 x (RT + 0.5k)) + (VIN x 15 ns)}
(1)
The buck switch duty cycle is approximately equal to:
DC =
t_ON
t_ON + t_OFF
= t_ON x F_S
=
V_OUT
V_IN
(2)
When the load current is less than one half the inductor’s ripple current amplitude the circuit operates in
discontinuous conduction mode. The off-time is longer than in continuous conduction mode while the inductor
current is zero, causing the switching frequency to reduce as the load current is reduced. Conversion efficiency is
maintained at light loads since the switching losses are reduced with the reduction in load and frequency. The
approximate discontinuous operating frequency can be calculated as follows:
F_S
=
V_OUT2
x L1 x 1.19
R_L x R_T2
x
10²¹
where
• R_L = the load resistance
• L1 is the circuit’s inductor
(3)
The output voltage is set by the two feedback resistors (R_FB1, R_FB2 in the Block Diagram). The regulated output
voltage is calculated as follows:
V_OUT = 2.51V x (R_FB1 + R_FB2) / R_FB1
(4)
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