lm2773tlx National Semiconductor Corporation, lm2773tlx Datasheet - Page 8

• Manufacturer Part Number: lm2773tlx
• Description: Low-ripple 1.8v/1.6v Spread-spectrum Switched Capacitor Step-down Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation Description
OVERVIEW
The LM2773 is a switched capacitor converter that produces
a selectable 1.8V or 1.6V regulated output. The core of the
part is a highly efficient charge pump that utilizes fixed fre-
quency pre-regulation, Pulse Frequency Modulation, and
spread spectrum to minimize conducted noise and power
losses over wide input voltage and output current ranges. A
description of the principal operational characteristics of the
LM2773 is detailed in the Circuit Description, and Efficien-
cy Performance sections. These sections refer to details in
the Block Diagram.
CIRCUIT DESCRIPTION
The core of the LM2773 is a two-phase charge pump con-
trolled by an internally generated non-overlapping clock. The
charge pump operates by using external flying capacitors
C
LM2773 will operate in a 1x Gain, with the input current being
equal to the load current, when the input voltage is at or below
3.5V (typ.) for 1.8V mode or 3.3V (typ.) for 1.6V mode. At input
voltages above 3.5V (typ.) or 3.3V (typ.) for the respective
voltage mode selected, the part utilizes a gain of 2/3x, result-
ing in an input current equal to 2/3 times the load current.
The two phases of the switched capacitor switching cycle will
be referred to as the "charge phase" and the "discharge
phase". During the charge phase, the flying capacitor is
charged by the input supply. After half of the switching cycle
[ t = 1/(2×F
In this configuration, the charge that was stored on the flying
capacitors in the charge phase is transferred to the output.
The LM2773 uses fixed frequency pre-regulation to regulate
the output voltage to 1.8V during moderate to high load cur-
rents. The input and output connections of the flying capaci-
tors are made with internal MOS switches. Pre-regulation
limits the gate drive of the MOS switch connected between
the voltage input and the flying capacitors. Controlling the on
resistance of this switch limits the amount of charge trans-
ferred into and out of each flying capacitor during the charge
and discharge phases, and in turn helps to keep the output
ripple very low.
When output currents are low (<40mA typ.), the LM2773 au-
tomatically switches to a low-ripple Pulse Frequency Modu-
lation (PFM) form of regulation. In PFM mode, the flying
capacitors stay in the discharge phase until the output voltage
drops below a predetermined trip point. When this occurs, the
flying capacitors switch back to the charge phase. After being
charged, the flying capacitors repeat the process of staying
in the discharge phase and switching to the charge phase
when necessary.
The LM2773 utilizes spread spectrum operation to distrubute
the peak radiated energy of the device over a wider frequency
band, reducing electromagnetic interference (EMI). Spread
spectrum is used during all modes of operation for the
LM2773.
EFFICIENCY PERFORMANCE
Charge-pump efficiency is derived in the following two ideal
equations (supply current and other losses are neglected for
simplicity):
In the equations, G represents the charge pump gain. Effi-
ciency is at its highest as G×V
1
and C
E = (V
2
to transfer charge from the input to the output. The
SW
OUT
) ], the LM2773 switches to the discharge phase.
× I
OUT
) ÷ (V
I
IN
= G × I
IN
× I
IN
IN
OUT
approaches V
) = V
OUT
÷ (G × V
OUT
. Refer to
IN
)
8
the efficiency graph in the Typical Performance Character-
istics section for detailed efficiency data. The transition be-
tween the gain of 1x and 2/3x is clearly distinguished by the
sharp discontinuity in the efficiency curve.
SHUTDOWN AND VOLTAGE SELECT
The LM2773 is in shutdown mode when the voltage on the
enable pin (EN) is logic-low. In shutdown, the LM2773 draws
virtually no supply current. When in shutdown, the output of
the LM2773 is completely disconnected from the input. Inter-
nal feedback resistors pull the output voltage down to 0V
during shutdown.
The SEL pin sets the output voltage at either 1.8V or 1.6V. A
logic-low voltage on the SEL pin will place the output of the
LM2773 in the 1.6V mode, and a logic-high voltage on the
SEL pin will place it into the 1.8V mode.
There are 350kΩ pull-down resistors connected internally be-
tween the EN pin and GND and the SEL pin and GND.
SOFT START
The LM2773 employs soft start circuitry to prevent excessive
input inrush currents during startup. At startup, the output
voltage gradually rises from 0V to the nominal output voltage.
This occurs in 150µs (typ.). Soft-start is engaged when the
part is enabled.
THERMAL SHUTDOWN
Protection from damage related to overheating is achieved
with a thermal shutdown feature. When the junction temper-
ature rises to 150°C (typ.), the part switches into shutdown
mode. The LM2773 disengages thermal shutdown when the
junction temperature of the part is reduced to 140°C (typ.).
Due to the high efficiency of the LM2773, thermal shutdown
and/or thermal cycling should not be encountered when the
part is operated within specified input voltage, output current,
and ambient temperature operating ratings. If thermal cycling
is seen under these conditions, the most likely cause is an
inadequate PCB layout that does not allow heat to be suffi-
ciently dissipated out of the µSMD package.
CURRENT LIMIT PROTECTION
The LM2773 charge pump contains current limit protection
circuitry that protects the device during V
where excessive current is drawn. Output current is limited to
500mA (typ).
Application Information
RECOMMENDED CAPACITOR TYPES
The LM2773 requires 4 external capacitors for proper opera-
tion. Surface-mount multi-layer ceramic capacitors are rec-
ommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR,
typ.). Tantalum capacitors, OS-CON capacitors, and alu-
minum electrolytic capacitors generally are not recommended
for use with the LM2773 due to their high ESR, as compared
to ceramic capacitors.
For most applications, ceramic capacitors with an X7R or X5R
temperature characteristic are preferred for use with the
LM2773. These capacitors have tight capacitance tolerance
(as good as ±10%) and hold their value over temperature
(X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to
85°C).
Capacitors with a Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2773. These
types of capacitors typically have wide capacitance tolerance
OUT
fault conditions
≤
15mΩ