Datasheet

F
S
=
V
OUT
2
x L1 x 1.18 x 10
20
R
L
x (R
ON
)
2
DC =
t
ON
t
ON
+ t
OFF
=
V
OUT
V
IN
F
S
=
V
OUT
1.3 x 10
-10
x R
ON
LM34910C
SNVS517B –MAY 2007–REVISED MARCH 2013 www.ti.com
Functional Description
The LM34910C Step Down Switching Regulator features all the functions needed to implement a low cost,
efficient buck bias power converter capable of supplying 1.25A to the load. This high voltage regulator contains a
55V N-Channel buck switch, is easy to implement, and is available in the thermally enhanced WSON-10
package. The regulator’s operation is based on a hysteretic control scheme, and uses an on-time control which
varies inversely with V
IN
. This feature allows the operating frequency to remain relatively constant with load and
input voltage variations. The hysteretic control requires no loop compensation resulting in very fast load transient
response. The valley current limit detection circuit, internally set at 1.25A, holds the buck switch off until the high
current level subsides. The functional block diagram is shown in Typical Application Circuit and Block Diagram.
The LM34910C can be applied in numerous applications to efficiently regulate down higher voltages. Additional
features include: Thermal shutdown, V
CC
under-voltage lockout, gate drive under-voltage lockout, and maximum
duty cycle limiter.
Hysteretic Control Circuit Overview
The LM34910C buck DC-DC regulator employs a control scheme based on a comparator and a one-shot on-
timer, with the output voltage feedback (FB) compared to an internal reference (2.5V). If the FB voltage is below
the reference the buck switch is turned on for a time period determined by the input voltage and a programming
resistor (R
ON
). Following the on-time the switch remains off for a minimum of 280 ns, and until the FB voltage
falls below the reference. The buck switch then turns on for another on-time period. Typically, during start-up, or
when the load current increases suddenly, the off-times are at the minimum of 280 ns. Once regulation is
established, the off-times are longer.
When in regulation, the LM34910C operates in continuous conduction mode at heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode current always flows
through the inductor, never reaching zero during the off-time. In this mode 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 operating frequency is approximately:
(1)
The buck switch duty cycle is equal to :
(2)
In discontinuous conduction mode current through the inductor ramps up from zero to a peak during the on-time,
then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB
falls below the reference - until then the inductor current remains zero, and the load current is supplied by the
output capacitor (C2). In this mode the operating frequency is lower than in continuous conduction mode, and
varies with load current. Conversion efficiency is maintained at light loads since the switching losses reduce with
the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as
follows:
where
• R
L
= the load resistance (3)
The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as
follows:
V
OUT
= 2.5 x (R1 + R2) / R2 (4)
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