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© 2006 Semtech Corp.

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SC250

PRELIMINARY

POWER MANAGEMENT

Applications Information

SC250 Detailed Description

The SC250 adaptive power controller is a step-down,

fixed frequency pulse-width modulated DC-DC converter

designed for use with RF Power Amplifiers (PAs) in

CDMA and WCDMA handsets and modules. The SC250

output is used to supply DC power to the PA rather than

connecting the DC input pin directly to the battery supply.

A substantial system power efficiency improvement can

be achieved by allowing the system controller to adaptively

adjust the DC power to the PA, reducing the total power

consumption of the device when in low-power mode. To

improve efficiency at all RF output gain settings, the PA

supply voltage is adjusted in a linear fashion, minimizing

PA supply headroom and losses.

A consequence of using the SC250 to power the PA, rather

than using a linear regulator or direct connection to the

battery, is that less current is needed. Reduced current

consumption results in more talk-time for the handset.

Operation Modes

The SC250 output voltage is dependent on the V

control voltage, defined by the following relationship:

V

In a typical PA system application, the system controller

determines what output power level is needed from the

PA and adjusts the VDAC voltage to match the required PA

headroom for optimized efficiency.

Pass-Through Mode

When the VDAC voltage reaches 1.36V, the SC250

enters pass-through mode. If the demanded output

voltage is within 430mV of the input voltage, the SC250

automatically enters pass-through as this exceeds the

maximum controlled duty cycle of the power converter.

In pass-through mode, the device enables an internal P-

Channel MOSFET that bypasses the converter, connecting

the output directly to the input. The R

extremely low, so there is little voltage drop across the part.

Pass-through allows the lowest insertion loss possible

between V

maintaining maximum efficiency under these conditions.

Bias Supply Output

In addition to the main output, the SC250 also provides a

low current LDO output that can be used as a bias supply

for power amplifiers. This output provides a regulated

2.85V with output current capability up to 20mA. The

2.85V output is guaranteed for input supply voltages in

excess of 2.95V.

Protection Features

The SC250 provides the following protection features:

Thermal shutdown

Current limit

Under-voltage lockout

Thermal Shutdown

The device has a thermal shutdown feature to protect the

device if the junction temperature exceeds 150°C. In

thermal shutdown, the PWM drive is disabled, effectively

tri-stating the LX output. The device will not be enabled

again until the temperature reduces by 10°C.

Short-Circuit Protection

The PMOS and NMOS power devices of the buck switcher

stage are protected by current limit functions. In the case

of a short to ground on the output, the LX pin will switch

with minimum duty cycle. The duty cycle is short enough to

allow the inductor to discharge during each cycle, thereby

preventing the inductor current from “staircasing.”

The pass-through PMOS is also protected by a current

limit function. When the part is first enabled in pass-

through, the output capacitor charges up with a large

surge current. This surge current is internally limited for

protection purposes, but the limit is set high enough to

meet fast start-up times. In order to protect against a

short-circuit condition and to allow the transient response

time, an internal timer allows the part to operate under

current limit conditions for a maximum of 64 cycles of the

internal clock (1MHz typical). If the short-circuit conditions

persists, the pass-through PMOS will turn off for 1ms,

after which the first timer is restarted. This allows the part

to manage thermal dissipation while giving it the ability to

recover when the fault condition is removed.

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